At present, Fighter stat is purely for fighter pilots, it increases their attack and defense against other aircraft. It also provides defense against AAW for Fighters.
Bomber is the same, but for bomber pilots, increasing defense against enemy aircraft and their ability to attack enemy aircraft. Again, it also increases defense against AAW.
The Aircraft stat is the same again, but for use by Recon pilots.
To answer your question though, to get the most ability in the end you shouldnt promote past the original bomber promotion, since although you gain an increase in growth, the rate your sailors crew size increases is reduced, which results in a net loss of ability.
