Decreased grind - lines are limited to 2 vehicles required per rank to bring down the grind to reasonable levels.
Consistent themes throuought lines - eg. Heavy bomber line being limited to machines with only bombs and defensive armament to keep their gameplay consistent.
Order not by BR
Some vehicles in this tree are not ordered by their BR. Here are justifications for each instance:
P-47N-15 - Thunderbolts in their folder are ordered chronologically, as well as by increasing speed. I'm not sure why P-47N is lower than the P-47D-28 in the first place.
A-10A Late - Memes aside, Thunderbolt was introduced into service 10 years later than the A-7, and I think that the tech tree should reflect that. Also, I'm convinced that it will go up in BR once the custom weapon loadouts are introduced.
PB4Y-2 - Being later modification of the B-24 that forgoes bottom ball turret as well as big chunk of top speed in favour of extra firepower in the upper hemisphere, I think that this sidegrade to the Liberator should be optional.
Solid line from start the end consinsting of USAAF and USAF fighters.
Twin-Engine & Heavy Fighters /// Export Fighters
OS2U's are here only because there was an empty spot - I would not change anything about them, and they would still get unlocked by playing naval as they are now in game.
Twin-Engine / Heavy Fighters - line of twin engine fighters. The the P-47 series is added here, because the main fighter line is already quite full, and also because it carries heavy armament that makes it as capable striking force as the rest of this line. F3D-1 ends the line, as it is a all-weather night fighter - not an attacker, as Gaijin would suggest.
Export Fighters - also moved over from the main fighter line as it is quite full. Even though F-104 and F-5 served with the US, they served way more with other countries, which ties them toghether as coherent sub-line.
Solid line from start the end consinsting of carrier-based aircraft operated by the USN.
This line is quite a mixed bag of various strike aircraft and torpedo bombers. Defining characteristics here are powerful guns and/or heavy suspended armament.
This is a very general name for bombers which are also equiped with offensive gun armament. A-26's are thrown in there to fill the rank IV gap, even though they are more of an attacker than a bomber, they fit the playstyle well enough.
This last line consists of - as I'd like to call them: pure bombers. That means, no offensive gun armament. Bombs and defensive guns only. IMO this provides very consistent playstyle throughout the line. This is also the reason B-57's are split - one has guns, other one does not - this makes them play very differently, and someone who enjoys one playstyle but not the other should not be forced to research vehicle he does not want. Lastly, I'm aware of BR gap between ranks II and III in this line, but I think it is justified in order to provide consistent gameplay. Besides, already datamined PBM-5 will hopefully provide a decent filler for this gap.