"Missing" USAF/DOD Aircraft Designations (2024)

"Missing" USAF/DOD Aircraft Designations

Copyright © 2002-2024 Andreas Parsch

USAF/DOD Aircraft Designations - Where to Find Them

Beginning in 1919, the USAAS (US Army Air Service) assigned simple alphanumerical designationsto all its aircraft. The basic system, consisting of letters designating type or mission followedby a sequential number ("Design Number"), was continued by the USAAC, USAAF, and USAF, even if the used type designationletters changed significantly several times in history. In 1962, this designation system became mandatoryfor use by all U.S. military services. The current designation system is explained in detail on this site in the articleabout Current Designations Of U.S. Military Aircraft, and for details about the earlier designation systemsyou may refer to this Wikipedia article.

There are (at least) two publicly available printed sources which attempt to givecomplete lists of USAF (and predecessors) and post-1962 Joint aircraft designations. Thesesources are:

    [1] John M. Andrade: "U.S. Military Aircraft Designations and Serials", Midland, 1979
    [2] Gordon Swanborough, Peter M. Bowers: "U.S. Military Aircraft since 1908", Putnam, 1989

For the latest designators, the references of choice are

    [3] Department of Defense: DOD 4120.15-L "Model Designation of Military Aerospace Vehicles", 1998and 2004
    [4] Addendum to DOD 4120.15-L (allocations between August 1998 and October 2018,acquired by the author from official sources)

With these sources as a reference, there are very few gaps left in the designation listings. Most of thesecan either be explained as deliberate gaps, or have been "closed" by further research. This article presents a list ofdesignations which cannot be found in any of the above sources, together with the current knowledge about the use/non-useof these designators. For the sake of completeness, designations which are already explicitly explained as "skipped" in [1] or [2]are also included.

Note: For each current series, an entry labeled Next Number is providedto indicate which design number will be (or should be!) allocated next in that series.

List of Designations "Missing" in Primary Sources

A (Attack), 1924-1948

A-1 was not assigned in the attack series, because the Cox-Klemin XA-1 (designated in the pre-1924 A-for-Ambulance series)was still in existence when the first attack aircraft (Douglas XA-2) was designated in 1924. There was also a Fokker A-2ambulance, but this was already out of service in 1924.

A (Attack), 1962-today

The designation A-8 was never used as such, because the "A-8" slot is incorrectly used by theBAE/McDonnell Douglas AV-8 Harrier VTOL aircraft. For moreinformation about the AV-8 designation, see article aboutNon-Standard DOD Aircraft Designations.

Officially, the A-11A designation was reserved but never assigned (for unknown reasons) in the 1986/88 time frame. There are a few rumours about a secret"A-11 Astra" stealth attack aircraft, but these are unconfirmed, and probably have no basis in reality. Even if such an aircraft actually existed,the use of the A-11 designator for it would be extremely unlikely.

Not used for superstitious reasons (see F-13).

The designation A-14A was briefly reserved to an Embraer A-29B Super Tucano evaluated by the US Navy. But the MDS was changed toA-29B (to match the Brazilian designation) before the aircraft was taken up. Therefore the A-14 slot will remain effectively unassigned.

Next Number
The next available design number in the A-series is A-15.

B (Bomber), 1924-1962

The proper pre-1962 B-series ends with B-70. Although the number 71 in the designation SR-71 followed on from B-70, the SR-71was never planned to be a bomber, and the designation B-71 was never reserved, let alone allocated, to it. For some backgroundinformation on the SR-71 designation, see article aboutNon-Standard DOD Aircraft Designations.

B-72 and higher:
The missile numbers SM-68 to GAM-87 are included in the B-series of sources [1] and [2], with gaps at B-74, B-79, B-81, B-82, B-84,B-85 and B-86. However, this is strongly misleading, because only missiles TM-61, SM-62, GAM-63, SM-64, SM-65 and GAM-67 did actuallycarry B-designations originally. The numbering sequence for missiles forked from the B-series at #68, so any missiles numbered 68and higher never used a B-designation. All of the gaps (plus higher numbers up to 92) were indeed allocated to missiles and rockets.For details about these missile designations, see article aboutPre-1963 Designations Of U.S. Missiles And Drones.

B (Bomber), 1962-today

Next Number
The next available design number in the B-series is B-3. The B-3 designation has been used informally to refer to proposed futurebomber concepts, but has never been allocated. In 2016, the designation B-21 was allocated to the LRS-B program. With this allocation,the use of a numbering sequence for bombers has effectively ended.

C (Transport), 1924-1962 / 2005-today

The C-13 designation was never assigned. This may be the first example of the skipping of -13 for superstitious reasons.Although -13 was not skipped in any other pre-1962 series, this has now become standard practice since the omission of F-13in the post-1962 fighter series (see F-13).

The C-136 designation had originally been skipped and reserved for use by the US Navy (see also H-36/38). The slot was laterused by the USAF, when it allocated the designation YC-136 on 19 July 1956 to a proposed modification of theFairchild C-123B Provider. The YC-136 was tobe a single prototype aircraft to test various improvements for the C-123B design.The YC-136 was apparently cancelled before it was built.

The C-138 designation had originally been skipped and reserved for use by the US Navy (see also H-36/38). Later, it wasplanned for allocation to the Fairchild F-27(license-built Fokker F-27 Friendship), if that type were acquired by theUSAF. This did not happen, and therefore the C-138 designation remained unused. It is not known, why a designation was reserved forthe F-27 before any firm plans to acquire the aircraft were actually made.

The designation SC-139 was requested on 18 October 1954 for a variant of theLockheed P2V-7 Neptune, but the designationwas disapproved on 23 November 1954. C-139 was not reassigned to another project.

The pre-1962 C-series ends at C-142. The XC-143 designation was requested for theCurtiss Model 200, but the requestwas turned down on 12 February 1962. Instead, the aircraft became the X-19, and still later (on 18 September 1962) the X-19A.

In December 2005, the pre-1962 C-series was revived, when the designation C-143A was allocated to aBombardier (Canadair) "Challenger" 604aircraft, which had been acquired by the U.S. Coast Guard. Unofficial information indicates that there has been some DOD-internalconfusion if C-42 (the nominal next number in the post-1962 C-series) was really available for allocation. The result was the continuationof the older C-series, because the C-143 slot was definitely available (see also paragraph on C-42 in new C-series).

Next Number
The "old" C-series is currently used in parallel with the "new" one (it is unclear which one will be used for thenext allocation). The next available number in the "old" series is C-148.

C (Transport), 1962-today

Not used for superstitious reasons (see F-13).

The C-16 designation was never officially assigned to any aircraft. However, it was tentatively reserved no less than four times before itwas finally cancelled for good. The first reservation, dated 27 November 1973, was for "YC-16" for an unidentified Boeing aircraft, butthis reservation is marked as "not used". On 13 March 1975, the designation C-16A was reserved for thedeHavilland Canada DHC-6. However, thisaircraft was eventually designated as UV-18A, and C-16remained unused. On 17 December 1975, C-16 was again put "on hold" for the Air Force. This third reservation was officially cancelledon 30 April 1981, shortly after the fourth and final reservation for C-16 was made on 13 April 1981. The latter was for the"C-X" aircraft (which eventually became theC-17A Globemaster). A formal request for allocationof an MDS designation, forwarded on 3 September 1981 by the USAF Nomenclature Office to HQ USAF for approval, says:

    1. The attached letter requesting a Model Series Designatorfor the C-X Aircraft is forwarded for consideration andapproval.

    2. We do not recommend assignment of C-17 as requested insubject letter.

    3. This office recommends assignment of C-16A to this Aircraftas "16" is the next available number in the Cargo Aircraftcategory. C-16 has been on reservation for this aircraftsince April and skipping this number is in conflict withAFR 82-1, paragraph 3g.

However, only one day later this letter was cancelled and replaced by the following:

    1. Disregard ASD/ENESS letter, 3 Sep 1981, same subject.

    2. The attached letter requesting a Model Series Designatorfor the C-X Aircraft is forwarded for consideration andapproval.

    3. We concur with assignment of C-17 as requested in theattached letter.

    4. The designation "C-16" will have to be marked in the DoDmaster list of aircraft designations as "Not Used".

There was obviously a reason to skip C-16, but unfortunately it was not written down. It is reported, however,that the design number 16 was skipped because of "concerns over confusing the plane with the F-16 during the stressof high combat radio traffic".

Interestingly, when Boeing worked in the 1978/80 time frame on a four-engined YC-14 derivative for the C-Xcompetition, some drawings were labeled with "C-16". However, this was simply a Boeing in-house label reflecting the anticipateddesignation of the C-X, and not related to any officially reserved C-16 designation.

The designation C-30 was never officially assigned. It was reserved (most probably after a verbal request at some time in the 1988/89time frame) for the USAF office with symbol "SAF/AQQX" (SAF/AQQ is the office symbol for the Assistant Secretary of the Air Force(Acquisition), Directorate of Global Reach; AQQX designates the Programs, Budget & Congressional Division of AQQ). Nomenclaturerecords do not contain any written request, let alone official allocation of the C-30 designation. Neither are there any releasablerecords on "C-30" at SAF/AQQX. The reservation was most likely dropped, but usage for a classified purpose can't be ruled out completely.

The "KC-30" label, which was used by Northrop Grumman for their proposed tanker version of the EADS/Airbus A330, isnot an official DOD designation. The same is true for the KC-30A designation used by the Royal Australian Air Force for itsA330-based tankers.

The C-34 designation was never assigned, because it was skipped at the request of the U.S. Army when the latter requesteda designation for what became the UC-35Aaircraft. On 9 February 1996, the Army's Product Manager for Fixed Wing Aircraft requested a designation for said aircraft:

    1. An award was made under subject contract on 26 January 1996 for purchase of Cessna Citation Ultra Model 560 aircraft.We request that your office initiate action to obtain an official Mission Design Series (MDS) designation of UC-35A forthese aircraft. [...]

    2. We are requesting the -35 designation which is one number out of sequence to avoid confusion with the Beechcraft(now Raytheon) T-34 series aircraft. Also, we want to avoid use of the -47 designation previously assigned to two CessnaCitation models (T-47A and OT-47B) because it has recently been corrupted by assignment of the designation C-47T to astretched turboprop powered modification of the World War II C-47 aircraft.

On 19 March 1996, the request was forwarded by the USAF Nomenclature Office to HQ USAF, which in turn approved the skipping ofC-34.

At some time in the March/May 1996 time frame, the designation YFC-36A was reserved for the U.S. Air Force. The only available detailis that this aircraft was to be a four-engined type. However, the "YFC" prefix (cargo aircraft converted to a prototype interceptor)is very unusual, and (together with the 1996 time frame and the "4 engines") strongly suggests that YFC-36A was temporarily reservedfor what became the YAL-1A "Airborne Laser" aircraft.For more information on the non-standard YAL-1A designation, see article aboutNon-Standard DOD Aircraft Designations.

Unclassified DOD nomenclature records contain no data at all on C-39, not even a non-descript reservation as is the case fora few other numbers, e.g. A-11 and C-30. There is one report stating that C-39A was the originally planneddesignation for what eventually became the U.S. Navy's C-40A Clipper.A possible reason would be that C-39 was skipped to avoid confusion with the CT-39 still in Navy service. However, available documentationdoes not support this report. The request by the Naval Air Warfare Center Aircraft Division for a new MDS for the aircraft, dated 10February 1998, doesn't request any specific model number. The letter simply shows "C-___A", with the number to be assigned by the USAF.The request was approved by HQ USAF with the allocation of model number 40. Apparently no specific reason was given by HQ USAF to theNomenclature Office why C-40A was allocated. However, given the skipping of V-14 and C-34 for a very similar reasons, it's not unlikelythat the "confusion with CT-39" report is indeed true, even if this reason was not recorded in any public documents.

The C-42 designation was not assigned. There is a very popular small sport plane, which is marketed world-wide under the "C42"(or "C-42") label: the German Comco Ikarus C42. A DOD sourcesaid unofficially, that there was some concern about potential legal issues if C-42 would be used for an MDS, and that thisnumber was therefore skipped.

C-43, C-44
The designations C-43 and C-44 were skipped to avoid potential confusion with the existing T/CT-43 and T-44 designators.According to unofficial information from DOD, there is an informal policy in effect to avoid duplication of "well-known" numbers.However, C-45 was not skipped (KC-45A is the official MDS for the KC-X tanker program), and it remains unclear why T-45should be any less "well-known" than T-43 and -44. One possible explanation is that both the T-43 (Boeing 737) and T-44(Raytheon/Beech King Air) are transport-type airframes (the T-45 is a two-seat jet), and that it was therefore avoided to assignnumbers 43 and 44 to other transport aircraft as well.

Next Number
The next available design number in the C-series is C-47. However, the designation C-47T (a turboprop-powered upgrade of the oldC-47 Skytrain transport of WW II vintage) was still listed as a non-retired MDS as of late 2018. Therefore it is not unlikelythat the C-47 slot will be skipped for the next C-series allocation.

E (Electronics), 1962-today

The E-7 designation was reserved in August 1981 and cancelled in January 1982. It was a Boeing 707 which was then allocated the basicdesignation C-18 (with the E-7 becoming the EC-18)due to its possible multi-mission roles.

The Royal Australian Air Force operates the Boeing 737 AEW&Caircraft as the E-7A Wedgetail, but this is not a U.S. DOD designation. However, the USAF has firm plans to procurevariants of the Wedgetail as replacement for its E-3 Sentry AEW fleet. If these plans are implemented, it's almostcertain that the Air Force will use E-7 as the aircraft's official MDS designation.

Next Number
The next available design number in the E-series is E-12.

F (Fighter), 1948-1962

This designation is usually attributed to the Bell Model D-188 supersonic VTOLfighter project of the late 1950s. However, while Bell and/or the ARDC (Air Research and Development Command) twice requested theYF-109 designation for this project (in January and October 1958), both requests were officially turned down by HQ USAF.

A few years earlier (most likely in 1955), F-109 was also requested for the two-seat interceptor version of the F-101 Voodoo(which became the F-101B in the end). However, this request (which is somewhat inexplicable anyway, because the next number in line atthat time was F-108) was apparently cancelled at an early stage, because no written documentation about it exists in USAF nomenclaturerecords (the only available first-hand source is a preliminary "Standard Aircraft Characteristics" sheet). Still other reports linkthe F-109 designation to a proposed operational version of the Ryan X-13 Vertijet VTOL demonstrator, but there is no firm evidenceavailable for this claim.

All said, F-109 was never officially allocated to any fighter project.

F-112 through F-116
The pre-1962 F-series ends with F-111. However, ever since the designation of the Lockheed Nighthawk stealth fighter wasannounced as F-117A, there has been speculation about the "missing" numbers F-112 through -116. It is now known (althoughit has never been officially acknowledged by the Air Force) that at least some of these numbers have been assigned tosecret projects of all sorts, including Soviet aircraft secretly acquired and tested by the USAF. However, the assigments werenot made in sequence, so that they can't be regarded as a proper continuation of the pre-1962 F-series. For more information,see article on Cover Designations for Classified USAF Aircraft.

F (Fighter), 1962-today

The designation F-13 was definitely not used. The Navy's F-14 Tomcatfighter would have received this number, but F-13 was rejected by Grumman and/or the Navy (obviously for superstitious reasons). Sincethat time (late 1960s), it is an unwritten rule that "-13" designators are always skipped. This eventually also led to the omission ofA-13, C-13, G-13, Q-13 and V-13.

The F-19 designation was never assigned. The official explanation by DOD was "to avoid confusion with MiG-19", which was generally regardedas very implausible (because so far no numbers had been skipped to avoid clashes with foreign designators). Therefore, it was muchspeculated whether F-19 was really skipped, and if so, for what real reason. One viable theory was that F-19 was originally allocated to(or at least reserved for) the F-117A Nighthawk, but eventuallynot used (see also article about Non-Standard DOD Aircraft Designations). The other main lineof reasoning was that Northrop specifically requested the F-20 designator for its then newTigershark (originally designated F-5G) to make itlook as "the first of a new fighter generation" (i.e., the "20" series).

The truth is in fact a combination of the second idea and the official line. The designation "F-19A" was indeed officially skipped atNorthrop's request. They wanted to redesignate the F-5G as F-20A, because they preferred an even number. The Soviet competitors inthe export fighter market of the early 1980s all used odd numbers, and Northrop wanted to stand out from these. So the official"confusion with MiG-19"-story is in fact more or less close to the truth, although the phrase is misleading. Nobody would"confuse" an "F-19A" with a MiG-19, especially because the latter was obsolete anyway at that time. To say it again, Northropdidn't want to avoid "confusion" with MiG-19 in particular, but to use an even number to stand out from all the Soviet odd ones.The F-20A designator was approved despite official recommendation by the USAF Standards Branch (at that time responsible fornomenclature assignments) to follow the regulations and use "F-19A".

The facts are documented by several letters exchanged between various USAF/DOD offices during the process ofrequesting and assigning the F-20A designator to Northrop. On 28 October 1982, HQ Aeronautical Systems Divison, USAF (apparentlyhandling the F-5G program for the Air Force) wrote a letter to the USAF Standards Branch to request a new model number for theF-5G on behalf of Northrop Corporation. To quote the relevant part (remarks in [brackets] are by me):

    1. In mid 1981, Northrop Corporation's Intermediate ExportFighter candidate was designated the F-5G. Since that time,the F-5G has incorporated substantial changes in structure,engine and aircraft systems. Northrop Corporation believesthese changes would be best reflected by a model designationchange from F-5G to F-20A, "Tigershark". Northrop's reasonfor specifically requesting the model 20 designation is thatbeing an even number series [sic], it would be unique in theforeign market which typically sees odd numbered threatdesignators (MIG 19; MIG 21; MIG 23).

On 1 November 1982, this request was in turn forwarded by the Standards Branch to USAF HQ in the Pentagon for approval.However, it was clearly stated that the designation should be "F-19A" instead, to follow existing regulations:

    1. The attached request [see quote above] is forwarded for yourconsideration and approval of a new Mission-Design-Series (MDS)designator.

    2. MDS designator F-5G was approved for Northrop's IntermediateExport Fighter candidate in May 1981. Based on the informationcontained in the subject letter, the aircraft has been changedsufficiently from the original F-5G configuration to warrantassignment of a new MDS as requested.

    3. Our records indicate that -19 is the next number availablefor assignment in the "F" series and to comply with AFR 82-1paragraph 3f we feel that F-19A should be assigned to thisaircraft.

    4. The popular name "Tigershark" has not been approved at thistime and should not be listed in DODL 4120.15 [DOD's official listing of approvedaerospace vehicle designations] until an MDS hasbeen assigned to this aircraft. We will take action to obtainapproval of the popular name when an MDS has been established.

    5. Our recommendations for entry into DODL 4120.15 are asfollows:

    a. [MDS] F-19A
    b. [Manufacturer] Northrop
    c. [Popular Name] unassigned

(The copy of the letter has a hand-written note at the bottom, saying "Dissapproved [sic] See F-20A folder".)

On 17 November 1982, HQ USAF approved the F-20A designation (but not yet the Tigershark name), apparentlyignoring the Standards Branch's advice, and introduced the misleading "confusion with MiG-19" phrase:

    1. Redesignation of the Northrop Corporation's intermediateexport fighter from F-5G to F-20A is approved. Substantialchanges in design and capability warrant a different basicdesign number. Northrop's concern for potential confusion withthe MIG-19 in their foreign markets is a sound basis for bypassingthat number.

    2. The assignment of the popular name "Tigershark" is beingworked through public affairs channels. [...]

    3. Please advise ASD and Northrop of the F-20A designation andthe status of the popular name request.

As a side note, the name Tigershark for the F-20A was eventually approved on 30 March 1983. It hadoriginally been requested on 4 September 1981 for the F-5G, but was then rejected "due to a proliferationof popular names for the F-5 aircraft series and the speculative nature of the F-5G venture" [USAF quote].

Next Number
The next available design number in the officially published F-series is F-24. However, the designationYF-24 has apparently been used (most likely "semi-officially") for an as yet undisclosed classified aircraft (referenced in theofficial USAF biography of Colonel Joseph A. Lanni).And after the allocation of the out-of-sequence F-35 designation to the JSF (see article aboutNon-Standard DOD Aircraft Designations), it's also possible that a future mannedfighter (e.g. the manned NGAD component) would be designated F-36.

G (Glider), 1962-today

TG-1A was allocated to theSchweizer SGS 2-25 sailplane used by the Air Force Academy.

TG-2A was allocated to theSchweizer SGU 2-22E sailplane used by the Air Force Academy.

TG-3A and TG-3B were allocated to theSchweizer SGS 1-26B and SGS 1-26E sailplanesused by the Air Force Academy.

TG-4A was allocated to theSchweizer SGS 2-33A sailplane used by the Air Force Academy.

TG-5A was allocated to theSchweizer SGS 2-32S sailplane used by the Air Force Academy.

TG-6A was allocated to theSchweizer SGS 1-34 sailplane used by the Air Force Academy.

Not used for superstitious reasons (see F-13).

Next Number
The next available design number in the G-series is G-18.

H (Helicopter), 1948-today

H-36, H-38, H-44
In the early 1950s, some even numbers in the C-series (C-134/136/138) and H-series (H-36/38) of USAF aircraft designations wereset aside for use by the US Navy, apparently in an attempt to establish a joint designation system for some type categories.However, this scheme was very short-lived, and had been abandoned by 1955. There is no evidence that any of the three C-seriesnumbers or H-36 was ever actually used by the Navy, but the case is a bit more confusing for H-38. The USAF aircraft serial54-4047 is listed in USAF records as a "Sikorsky H-38", allocated to a Navy-led MAP (Military Assistance Program). There areindications that the serial refers to a "static airframe", but otherwise no further information whatsoever about this H-38 hasbeen found so far.

In any case, by 1959 the USAF apparently regarded the H-36 and -38 slots as unused. A letter from an Air Force Intelligenceoffice (AFCIN-4F), dated 18 May 1959, requested the allocation of H-36, H-38 and H-44 (which was the next available number atthat time) to three secret projects called LONG EARS (H-36), SHORT TAIL (H-38) and BIG TOM (H-44). This letter explicitlymentions that H-36 and -38 had been "set aside but never used by the Navy". All three designations were approved in August 1959.The USAF aircraft serial 59-5926 is listed as a "Bell H-36", but otherwise no information whatsoever is available aboutthe three projects or the associated helicopters.

There is a rumour that the Sikorsky XV-2 (Model S-57) design (which was cancelled around 1954/55 before a prototype wasbuilt) was originally designated XH-36. While this doesn't sound implausible (the XV-1 and XV-3 were both originally designated inthe H-series as XH-35 (→ XV-1) and XH-33 (→ XV-3), respectively), there is no indication whatsoever in very respectablesecondary sources (which all list the redesignations of the XV-1 and -3 designs) for an XH-36 → XV-2 redesignation.In fact, as is documented in the preceding paragraphs, the primary source (USAF nomenclature allocation letters) explicitlysays that H-36 had been skipped. So it appears that the association of the XV-2 with the XH-36 designation is in error.At best, it was unofficially discussed at some time early in the S-57 project to designate the aircraft as XH-36, butthere was no formal effort to reserve, let alone allocate, the H-36 designation for the project.

XH-42 was allocated to a Hughes Model 269 for the U.S. Army, which eventuallybecame the YHO-2HU. It was later developed as the TH-55A Osage.

H-45A was allocated in January 1962 to a secret Air Force FTD (Foreign Technology Division) project called STEP CHILD. USAF serial62-5980 is attributed to an "XH-45", but otherwise no information is available about the STEP CHILD program or the H-45 helicopter.

XH-49A was briefly allocated in 1962 to a Boeing-Vertol Model 107helicopter for the USAF. The designation was changed to XCH-46B before September 1962.

When the VH-71A designation was assigned to the Lockheed Martin US-101(winner of the VXX (Presidential Helicopter) competition), the design numbers 69 and 70 in the H-series were skipped (H-70 has since beenallocated to the Bell ARH). As has been confirmed by the VXX Program Office, the number "69" was regarded as embarrassing, because thatnumber is also known as a synonym for a certain sexual practice. According to USAF/XPPE, H-69 will not be assigned in the futureto any helicopter.

Next Number
The next available design number in the H-series is H-74.

K (Tanker), 1955-1977

The K-series was officially reserved for tanker aircraft between 1955 and (at least) 1977, but no designationwas ever allocated in this series. All U.S. tankers were derivatives of other aircraft, and therefore the K-series wascancelled at some time between 1977 and 1986, and only the K-for-Tanker modified mission symbol remains. A purpose-builttanker would most probably receive a "KC" designation in the C-series.

L (Laser), 1997-today

Next Number
The next available design number in the L-series is L-2.

O (Observation), 1962-today

The designation ZO-4A (indicating a design in the planning stage) was reserved by the USAF on 28 May 1969. The O-4 was to be a quietobservation aircraft, similar in concept to the Lockheed YO-3A. It seems that one candidate (possibly the only one) for the O-4 requirementwas the Wren 460QB, a modified Wren 460B (itself a highly modified derivative of the Cessna 182).In 1969, the USAF proposed the purchase of 28 of these aircraft. However, the O-4 program was terminated in the early 1970s because of budget restrictions.

Next Number
The next available design number in the O-series is O-7.

P (Pursuit), 1924-1948

P-73 was never officially assigned. The generally accepted reason is that the USAAF skipped P-73 and P-74 because theFisher Division of General Motors requested the "special" number 75 for theirXP-75 Eagle escort fighter. However, the XP-73designation was at least proposed for (but not actually allocated to) theHughes D-2, which was eventually designated as XA-37.

P-74 was never assigned (for details, see P-73).

P (Patrol), 1962-today

The designation P-1 was definitely not used, apparently out of convenience to use numbers P-2/3/4/5 to redesignate theP2V, P3V, P4Y, and P5M, respectively (see also article on Aircraft Redesignations in 1962).

The designation P-6 was never assigned, even if it is frequently quoted as post-1962 designation of theMartin P6M Seamaster. However, official DOD aircraft nomenclatureforms of 1962 say "P-6 skipped for the Navy". This was most likely done to avoid any future confusion with the P6M.

Next Number
The next available design number in the P-series is P-10.

Q (Target Drone), 1948-1963

In late 1953, the WADC (Wright Air Development Center) requested the redesignation of the QB-17 drone to Q-7A, Q-7B and Q-7C (presumablyrepresenting different drone configurations, possibly the QB-17G, -17L and -17N). The stated reason were problems and delays in theproduction and support of the drones, because they were too easily confused with unmodified B-17 aircraft. However, HQ ARDC(Air Research & Development Command) did not concur with this reasoning, and turned the request down in March 1954. Becausethe Q-7 designation was not re-used otherwise, the #7 slot in the Q-series remained unassigned.

In 1948, a few remaing Culver PQ-8A Cadet drones were possibly redesignated as Q-8A. However, these were discarded soon after, andthe Q-8 designation became available for use in the regular Q sequence.In late 1953, the WADC (Wright Air Development Center) requested the redesignation of the QF-80 drone to Q-8A, Q-8B and Q-8C (presumablyrepresenting different drone configurations, possibly QF-80A, -80C and -80F). The stated reason were problems and delays in the productionand support of the drones, because they were too easily confused with unmodified F-80 aircraft. However, HQ ARDC (Air Research &Development Command) did not concur with this reasoning, and turned the request down in March 1954. Because the Q-8 designation was notre-used otherwise, the #8 slot in the Q-series remained unassigned.

In July 1959, the WADC (Wright Air Development Center) requested the allocation of the designation XQ-11 to a so called "F-108 AirborneTarget". This referred to a projected high-performance aerial target for evaluation of the whole F-108 interceptor weaponssystem (WS-202A). However, USAF Headquarters did not approve the request, stating that a specific designation for the target would notbe necessary at such an early stage of the program. The Q-11 designator was not used otherwise afterwards, and therefore the #11 slot inthe Q-series remained unassigned.

Q (UAV), 1997-today

Not used for superstitious reasons (see F-13).

Next Number
The next available design number in the Q-series is Q-29. In May 2023, the company Shield AI proudly posted in several of theirsocial media channels, that their V-BAT UAV has been "officially designated" as MQ-35A. It must be assumed, that thisis really a DOD allocation, but there is currently no public record of any Q-series design numbers from 29 to 34. Either all thesenumbers were assigned between 2022 and May 2023, but have yet to be mentioned in public (in which case the next available numberwould of course be Q-36), or MQ-35 is yet another out-of-sequence allocation.

R (Reconnaissance), 1948-1962

The R-series continued the old F-series, which had reached F-15 by 1947. Reference sources usually mention only a singlepost-1947 designation, the XR-16. However, when the USAF first planned to acquire its own batch of the Lockheed U-2,following the CIA's lead, it used the designation R-17 for the aircraft. In the end, this designation was dropped, andthe USAF aircraft were also designated U-2.

R (Reconnaissance), 1962-today

R-2 and higher:
The only official designation allocated in the proper sequence of the R-series so far is TR-1, and even this is anon-standard designation. The NASA'sLockheed ER-2 aircraft is listed as "R-2" in some sources,but this is an error. The "ER" stands for "Earth Resources", and the ER-2 designation was assigned by NASA and not by the DOD.The TR-3A designation, which was rumoured to be allocated to a secret stealth battlefield surveillance platform(which almost certainly doesn't exist), is purely fictitious.

Next Number
The next available design number in the R-series is R-2.

S (Anti-Submarine), 1962-today

The designation S-1 was definitely not used, apparently out of convenience to use number S-2 to redesignate the S2F(see also article on Aircraft Redesignations in 1962).

Next Number
The next available design number in the S-series is S-4.

S (Spaceplane), 1990-today

Next Number
The S-for-Spaceplane series is in potential conflict with the S-for-Anti-Submarine series. So far, the only spaceplane designationis MS-1A, which created no duplication, because the S-1 slot was not used in the anti-submarine series. Whether any new spaceplaneMDS would use the S-2 or the S-4 slot is unknown.

T (Trainer), 1962-today

T-4, T-5
Both designations were skipped to use T-6A Texan II for the new USAF/Navy JPATS Trainer to honour the AT-6/T-6 Texanof WW II fame. After the subsequent allocation of T-7, it's very unlikely that these gaps will be filled by later training aircraft.

T-48, T-49
A memo from HQ USAF (which confirms the allocation of the T-1A designation to the Beech Jayhawk) to the USAF Nomenclature Office,dated 28 February 1990, includes the following paragraph:

    Additionally, reserve MDS designations of T-48A and T-49A for the USAF and US Navy Joint Primary Aircraft TrainingSystem (JPATS) aircraft, respectively.

The designation YT-48A is generally associated with a proposed development of the Cessna T-37 as a replacement for theUSAF's cancelled T-46A. It seems that the USAF used their reserved T-series MDS for this proposal. The T-49A designation, which wasreserved for the Navy, was apparently never taken up. In the end, a common JPATS aircraft was selected, which was designatedT-6A Texan II (see also T-4/T-5 above).

During 2003, the designations T-48 and T-49 were newly assigned (see Addendum to DOD 4120.15-L).Therefore it seems that the earlier reservations had been cancelled, because a re-use of reserved but never allocated and later cancelleddesignations is actually "legal" in the MDS system.

The T-50A MDS designator is not allocated to any current U.S. military aircraft, but has been officially reserved for theLockheed Martin/KAI Golden Eagle military trainer, which is marketed under the "T-50" company designation.Although the U.S. military currently has no plans to procure the Golden Eagle, it was apparently regarded as too confusing tohave T-50A eventually assigned to a different model.

Next Number
The next available design number in the T-series is either T-55 or T-8, depending on whether the next number will be allocated in the sequence carried over from the pre-1962 numbering, or the new one started in 1962.

U (Utility), 1952-today

U-12, U-13, U-14, U-15
The designations U-12 through -15 were definitely not used, apparently out of convenience to use number U-16 to redesignate the SA-16(see also article on Aircraft Redesignations in 1962).I don't know why a 4-number gap was approved just to keep a single number unchanged.

Next Number
The next available design number in the U-series is U-29.

V (V/STOL), 1962-today

Not used for superstitious reasons (see F-13).

In May 1973, the U.S. Army and NASA requested an MDS designation for the forthcomingBell Model 301 tilt-rotor demonstrator aircraft.On 30 May 1973, the designation XV-14 was allocated. However, on 7 August 1973, a change of this designation wasrequested to avoid confusion with the Bell X-14B VTOL aircraft. To quote the most important parts of this request:

    3. It is requested that the XV-14 model designation be rescinded and a new model designation be assigned to theTilt Rotor Research Aircraft because of the likely confusion with the X-14B VTOL aircraft.

    4. The Tilt Rotor Research Aircraft will be delivered to, and research flights will be conducted at, the Ames ResearchCenter. Flight tests are also currently being conducted at Ames using another experimental aircraft, the X-14B.[...]

The letter continues to explain at some length how easily the two designations could be confused, especially becauseboth aircraft are built by companies both known shortly as "Bell" (Bell Helicopter Company XV-14 and Bell AerospaceCompany X-14B). Reference is made to the potential of serious errors if the service and maintenance recordsof the two aircraft are somehow mixed up. The letter ends with:

    [...] Equally important, however, is the unnecessary resultant confusion to the aviation R&D community in general, and tothose persons in Army and in NASA who are not familiar with these aircraft. This likely confusion is compounded by the fact thatthe X-14B has been undergoing evolutionary changes since 1959 (from the X-14 to the X-14A to the X-14B) and it is possible thatan XV-14 at Ames could be confused with a modification of the X-14, particularly as they are both VTOL aircraft.

The redesignation request was approved on 22 August 1973, and the aircraft became the XV-15.

XV-17 was assigned to the Army on 24 May 1973. No details are available.

V-19A was reserved on 25 February 1977 for the Navy, but the reservation was cancelled on 8 December 1980. A note on the nomenclaturerecord card says:

    Cancelled after contact with Tim Nichols (McDonald's [who apparently made the reservation in 1977] replacement) of NavAir. He canfind nothing on this reservation.

Given that not even the responsible Navy officer knew what the V-19A designation was intended for after only 3 years, it looks unlikelythat we will ever find out more about the "V-19".

The designation PV-21 was reserved on 15 December 1983 by the Navy for a blimp (non-rigid airship) with tilt rotors.In 1983/84, the USN conducted a "Patrol Airship Concept Evaluation Study" (PACES). This included flight-testing of an experimentaltilt-rotor blimp during mid-1983. The PV-21 designator (indicating a patrol mission) was possibly reserved either for the PACES demonstratoror a planned tactical development, like the Battle Surveillance Airship System studied as a follow-on project to PACES.

Next Number
The next available design number in the V-series is V-25.

X (Experimental), 1948-today

The X-23A designation is generally attributed to theMartin Marietta SV-5D PRIME unmanned lifting bodyreentry test vehicle, but available USAF nomenclature records show that X-23A was never actually assigned.On 16 November 1965, the designation X-23A was requested for the SV-5P[sic!] vehicle, which is known to have been designated as X-24Ain mid-1967 (see next paragraph). The vehicle description accompanying the designation request of 1965 clearly describes the SV-5P as alow-speed (Mach 2 to landing) manned lifting-body aircraft. However, in a letter dated 15 December 1965, the request was disapprovedfor the reason that the subject aircraft was unmanned (at that time, the aircraft designation system was still used as originallyintended in 1962, i.e. for manned aircraft only)! This appears to be very weird indeed, but apparently there was a severe misunderstandingregarding the nature of the research aircraft at the office which had to approve the designation.

In late 1966, the offices responsible for the USAF's lifting body reentry programs again pondered the question how to designatethe test vehicles. After a stillborn proposal to introduce a completely new designation category for gliding reentry vehicles,it was decided that the best way to go was to request the designations X-23A for the unmanned SV-5D PRIME and X-24A for themanned SV-5P. X-24A was accordignly requested and approved, but it appears that no actual request for X-23A was ever sentto the nomenclature office. Reasons are unknown, but maybe it was realized that an MDS request for an unmanned vehicle was futile,especially when the rejection of the 1965 request for X-23A explicitly said that unmanned aircraft need no designation. Whatever thereasons, the designation X-23A was never even requested for, let alone allocated to, the SV-5D PRIME vehicle.

X-39 was reserved on 23 April 1997 for the FATE(Future Aircraft Technology Enhancements) program of the AFRL (Air Force Research Laboratory). However, noformal written request to allocate X-39 to FATE was put forward, and therefore X-39 remained officially unassigned.

The designation X-52A was requested in 2006 for a program to testactive aeroelastic wing technology, but was disapproved because ofpossible confusion with the B-52 series. Instead the designation X-53A was allocated to the program.

The X-58 designation has been skipped. The designation XQ-58A was assigned to theKratos ValkyrieUAV built for the AFRL (Air Force Research Laboratory), and it is practically certain that the number 58 was chosen to fit intothe X-series even though an "XQ"-designated vehicle should get a number in the Q-series.

X-63, X-64
At the time of this writing (February 2024) there is no public reference to an X-plane designation between X-62 and X-65.

Just like X-58, the X-67 slot will almost certainly be skipped after the allocation of XQ-67A to theGeneral Atomics OBSS (Off-Board Sensing Station) UAV.

Next Number
The next available design number in the X-series is X-68.

Z (Lighter-than-Air), 1962-today

Next Number
The next available design number in the Z-series is Z-5.

Comments and corrections to: Andreas Parsch

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Last Updated: 9 February 2024

"Missing" USAF/DOD Aircraft Designations (2024)
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