GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
The Japanese Navy was similar with their naming convention, G4M for example:I believe the idea was to assign a letter consistent with the manufacturers name. The problem is that several aircraft manufacturers had the gaul to use the same first letter in the name of the company.
F4U - The company that produced the Corsair was Chance-Vought, but the 'C' was already taken by Curtiss, so it was designated with a 'U'.
F4F, F6F, F7F, F8F, etc. - Grumman produced many aircraft for the Navy, but 'G' was already the designator for Great Lakes and later Goodyear, so it was designated with 'F'.
SNJ - North American was the manufacturer, but 'N' was already assigned to the Naval Aircraft Factory, so it was designated with 'J'. As an aside, the 'S' was for Scout, and the 'N' was for Trainer, 'T' having already been used to designate a Torpedo carrying.
See, clear as mud...
G - land based attack bomber
4 - fourth type in "G" series
M - Mitsubishi
In regarda to the USN's system, it was established in the 1920's and several of the suffixes were reused over time.
Typically, the letter was assigned in the order it became available.
The Naval Factory's "N" suffix was in use and Berliner-Joyce's "J" suffix was no longer in use (1929-1935), so North American got it.
The "G" suffix was in use by Great Lakes between 1929 and 1935, but when it became available, Goodyear got it, because Grumman had been assigned "F" in 1931.
So in most cases, a manufacturer was assigned the next available slot unless it just happened that an available letter coincided with their name.
As far as the trainer designation, the "N" was selected when the system was established in 1922, so they did use "T" for torpedo, as the USN wanted their combat aircraft to have the most recognizable prefix. Trainers and other non-combat types had "second pick" designations.