Japanese aircraft designations are a highly confusing subject, since four
different systems were in use simultaneously in Japan, in addition to the
codenames used by the Allies.  The Japanese Army and Navy each used two
systems to identify the same aircraft, so a type used by both services
(there were a few) could have up to five different designations -- Japanese
Army Kitai number, Army type number, Navy designation code, Navy type
number, and Allied codename!

Just to confuse matters a bit further, a few types were known best by
nicknames that had no official status.  The Mitsubishi A6M fighter, also
known as the Carrier-Borne Fighter Type 0, had the official Allied codename
of "Zeke"; but it went down in history under the unofficial nickname used
by both sides:  "Zero".

The Japanese Army Air Force identified aircraft by "Kitai" (airframe)
numbers, which simply consisted of "Ki", a dash, and a number.  Originally
the numbers were a simple numeric sequence; later, some randomisation was
added, as a security measure.  Gliders received "Ku" ("Guraida") numbers
instead.  Subtypes or variants were indicated by Roman numeral suffixes, or
by various Japanese abbreviations (a common one was "Kai" (for "Kaizo"),
indicating a major modification).

In addition to Kitai numbers, most Army aircraft also received a second
designation in a parallel system based on role and the year of entry into
service.  Originally this was the last two digits of the year; 100 was used
for the Japanese year 2600 (1940), then the numbers were restarted from 1.

Imperial Japanese Navy aircraft received a designation code very similar to
those used by the US Navy.  This consisted of a letter to indicate the
aircraft's function, a sequential number to indicate a specific aircraft
type (unlike the USN system, the number 1 was left in), and a letter to
indicate the manufacturing company.  This was followed by a dash and a
number to indicate a subtype, plus an optional letter or letters for
further variations.

Function letters:

    A = Carrier-borne fighter
    B = Carrier-borne torpedo bomber
    C = Carrier-borne reconnaissance
    D = Carrier-borne dive bomber
    E = Reconnaissance seaplane
    F = Observation seaplane
    G = Land-based bomber
    H = Flying-boat
    J = Land-based fighter
    K = Trainer
    L = Transport
    M = Special-purpose seaplane
    N = Fighter seaplane
    P = Bomber
    Q = Patrol
    R = Land-based reconnaissance
    S = Night fighter

Some manufacturer letters:

    A = Aichi
    D = Showa
    K = Kawanishi
    M = Mitsubishi
    N = Nakajima
    P = Nihon
    V = Seversky
    W = Kyushu, Watanabe
    Y = Yokosuka

The IJN also used a parallel system based on role description and year
number, similar to (but independent of) the Army's, except that the year
2600 (1940) became 0 instead of 100.  This system was abandoned in 1943,
when it was decided that revealing the year of an aircraft's entry into
service might give useful information to the enemy.  Aircraft were then
given proper names instead.

Because the correct designations of Japanese aircraft were often not known
(and, as you've probably gathered by now, difficult to keep straight
anyway), the Allies assigned codenames to them. The basic rules for these,
not always followed, were:

    Bombers, dive bombers, reconnaissance aircraft, seaplanes, torpedo
        bombers -- Girls' names
    Fighters, reconnaissance seaplanes -- Boys' names
    Gliders -- Names of birds
    Trainers -- Names of trees
    Transport aircraft -- Girls' names beginning with "T"

The following list gives various designations for some of the more
important Japanese aircraft of WW2:

  Aichi D3A                 = Navy Type 99 Carrier-Borne Fighter    = "Val"
  Kawanishi H8K             = Navy Type 2 Flying-Boat               = "Emily"
  Kawanishi N1K1/2 Shinden                                          = "George"
  Kawasaki Ki-45 Toryu      = Navy Type 2 Heavy Fighter             = "Nick"
  Kawasaki Ki-61 Hien       = Navy Type 3 Fighter                   = "Tony"
  Kawasaki Ki-100           = Navy Type 5 Fighter
  Kyofu N1K                                                         = "Rex"
  Mitsubishi A5M            = Navy Type 96 Carrier-Borne Fighter    = "Claude"
  Mitsubishi A6M            = Navy Type 0 Carrier-Borne Fighter     = "Zeke"
  Mitsubishi F1M            = Navy Type 0 Observation Seaplane      = "Pete"
  Mitsubishi G4M            = Navy Type 1 Attack Bomber             = "Betty"
  Mitsubishi J2M Raiden                                             = "Jack"
  Mitsubishi Ki-15 Karigane = C5M                                   = "Babs"
  Mitsubishi Ki-21          = Army Type 97 Heavy Bomber             = "Sally"
  Mitsubishi Ki-30          = Army Type 97 Light Bomber             = "Ann"
  Mitsubishi Ki-46 Shitei   = Army Type 100 Reconnaissance Aircraft = "Dinah"
  Mitsubishi Ki-67 Hiryu    = Army Type 4 Heavy Bomber              = "Peggy"
  Nakajima B5N              = Navy Type 97 Carrier-Borne Bomber     = "Kate"
  Nakajima B6N Tenzan                                               = "Jill"
  Nakajima J1N Gekko                                                = "Irving"
  Nakajima Ki-27            = Army Type 97 Fighter                  = "Nate"
  Nakajima Ki-43 Hayabusa   = Army Type 1 Fighter                   = "Oscar"
  Nakajima Ki-44 Shoki      = Army Type 2 Fighter                   = "Tojo"
  Nakajima Ki-49 Donryu     = Army Type 100 Heavy Bomber            = "Helen"
  Nakajima Ki-84 Hayate     = Army Type 4 Fighter                   = "Frank"
  Yokosuka D4Y Suisei                                               = "Judy"
  Yokosuka P1Y Ginga                                                = "Frances"