The US Air Force (and its predecessor, the US Army Air Force) has used
several aircraft designation systems in its history.  The three most
important are the USAAF system adopted in 1924 and used through World War
II and up to 1948; the USAF system used from 1948 to 1962; and the
Tri-Service system adopted in 1962 to provide a common system for USAF, US
Army, and US Navy aircraft.  The three systems are similar enough that they
can conveniently be described together.

A designation consists of a letter (or set of letters) indicating the type
and mission of the aircraft, and a sequence number indicating a specific
aircraft within a category, separated by a hyphen.  The number may be
followed by a series letter to indicate a variant of an aircraft.  Most
aircraft are also given a proper name, but this is not part of the formal
designation.

Mission codes used in the USAAF system included the following:

    A  = Attack
    AG = Assault glider
    AT = Advanced trainer
    B  = Bomber
    BC = Basic combat
    BG = Bomb glider
    BQ = Guided bomb
    BT = Basic trainer
    C  = Cargo transport
    CG = Cargo glider
    CQ = Target control
    F  = Photographic reconnaissance
    FG = Fuel-carrying glider
    FM = Multiplace fighter
    G  = Gyroplane
    GB = Glide bomb
    GT = Glide torpedo
    JB = Jet-propelled bomb
    L  = Liaison
    O  = Observation
    OA = Observation amphibian
    OQ = Target
    P  = Pursuit
    PB = Biplace pursuit
    PG = Powered glider
    PQ = Manned target
    PT = Primary trainer
    R  = Rotorcraft
    TG = Training glider

These were sometimes modified by one of the following prefixes, indicating
a special status or modification:

    C = Cargo transport
    F = Photographic reconnaissance
    K = Ferret
    R = Restricted operations
    T = Trainer
    U = Utility
    V = Staff/VIP transport
    X = Experimental
    Y = Service test
    Z = Obsolete

The first version of a type had no series letter; the second was suffixed
with "A", the third with "B", and so on.  For obvious reasons, the letters
"I" and "O" are usually skipped.  For example, the B-29A is the second
version of the 29th bomber aircraft identified by the USAAF.

The USAF system (1948) was similar to the USAAF system; it retained the
three-part code, although the series letters now started with "A" for the
first version rather than the second.  The mission codes were rationalised
somewhat; "F" for "Fighter" replaced "P" for "Pursuit" (the existing
P-series aircraft being redesignated, and new aircraft receiving F-series
numbers continuing the old P-series), "H" for "Helicopter" replaced "R" for
"Rotorcraft", and "R" for "Reconnaissance" replaced "F" for "Photographic".
The "L" for "Liaison" code was subsumed by "O" for "Observation", and most
of the two-letter codes were combined into one (e.g.  a single "T" series
replaced the old "AT", "BT", and "PT").

The Tri-Service system (1962) underwent further changes, although it still
retained the basic scheme of the older systems.  The most important changes
were that the system now included Navy aircraft as well as USAF and Army,
and that most of the numeric sequences were restarted from 1, since some
were now well past 100 and were becoming unwieldy.

Starting from the central dash and moving *left*, the letter codes now
consist of up to four letters (although only the "basic mission" code is
mandatory, and I've never seen a real designation with more than three
letters).

(1) Vehicle type (optional; indicates something other than a conventional
fixed-wing aircraft):

    G = Glider
    H = Helicopter
    V = VTOL or STOL
    Z = Lighter than air (Z for Zeppelin)

(2) Basic mission:

    A = Attack
    B = Bomber
    C = Cargo transport
    E = Special electronics
    F = Fighter
    O = Observation
    P = Maritime patrol
    R = Reconnaissance
    S = Anti-submarine warfare
    T = Trainer
    U = Utility
    X = Research

(3) Modified mission (optional; indicates that a type originally designed
for the mission indicated by its "basic mission" code has been modified for
a different mission); includes the A, C, E, F, O, P, R, S, T, and U mission
codes, plus:

    D = Drone control
    H = Search and rescue
    K = Tanker (K for Kerosene)
    L = Cold weather
    M = Multi-mission
    Q = Drone
    V = VIP or staff transport
    W = Weather observation

(4) Status (optional; indicates any unusual status):

    G = Permanently grounded
    J = Temporary special test
    N = Permanent special test
    X = Experimental
    Y = Prototype
    Z = Planning

The sequence numbers are based on the vehicle type (if present) or the
basic mission.  For example, all helicopters (vehicle type "H") are
numbered in a single sequence regardless of the basic mission code, while
conventional aircraft (with no vehicle type code) follow separate sequences
for attack aircraft, bombers, transports, and so on.  There are a few
oddities here; for example, the AV-8 Harrier seems to have taken the number
8 slot in both the "A" and "V" sequences.  For some reason, the "T"
(trainer) sequence, last seen in the Cessna T-47 in 1984, was restarted
with the Beech T-1 Jayhawk in 1990.

The system has not been followed perfectly; exceptions include the A-37
Dragonfly (attack version of T-37 trainer; there was an AT-37, so the A-37
should have either continued the AT-37 designation or been given a proper
A-series number), F/A-18 Hornet (should have been just F-18, or possibly
AF-18), FB-111 (bomber version of the F-111; should have been BF-111),
SR-71 (the letters indicate "strategic reconnaissance", not an
anti-submarine modification, and the number is actually from the pre-1962
bomber series!), and a few others.