Interesting that they seem to have followed the British system in use at the same time...
"Fighters - general words indicating speed, activity, or aggressiveness"
"Bombers - Place names - (an inland town of the British Empire or associated with British history- not appropriate to the US, obviously)"
"Transports - counties or districts (of the British Empire)"
"Trainers - words indicating tuition and places of education"
The above from 'Names with Wings' Gordon Wansborough White, Airlife, 1995, ISBN 1 85310 491 4
cheers,
Robin.