[Be advised: This tech tree is still unfinished, unpolished and contains some obvious mistakes in the stat cards, which are also mostly unfilled. And I'm a little clueless about modern jets in their specifics.] Scroll down for the actual tech tree.

I focused on mainly the more practical prototypes that achieved some breakthrough within the propeller planes areas. As well as some genuine aircraft that are pretty unheard of. Tech tree battle ratings were re-evaluated due to modifications available for certain aircraft. I also tried to diversify the aircraft with more branches because I thought it would look cool...

I sort of put too much focus into the higher-end World War II aircraft, it's just that those intrigued me the most... Well that was the plan initially but I randomly decided to have fun with some of the late cold war era and modern aircraft too. That's why the branch headings may be confusing at first:

Fighters - Mainly the straightforward light-medium fighters and naval fighters.

Specialized Interceptors - High Altitude specialized aircraft. Later on (by that I usually mean once you get into the Cold War era stuff) they tend to be light fighters (which also count as interceptors I guess).

Universal Fighters - Inclined a bit towards speed, tougher, later on transitioning to strike fighters. (Yeah by the time i reached rank 6 i sort of realize my diversity branches have kinda failed but, again, my original goal was to stop at rank 4... or 5...).

Universal Strike Aircraft - Performance calibrated mainly for ground support firepower.

Specialized Strike Aircraft - Improvised or Specialized Air Defense fighters with focus on firepower and escort.

Strike Bombers - Frontline, brisk and medium, dive bombers, with various focus between protection and firepower or agility. Later on are formed from adapted strike fighters.

Universal Bombers - Medium or Heavy, Strategic bombers aimed at delivery performance. 

Overall you can kind of see how each sub-tree has a certain lack and later on they become aircraft that overcome the noticable shortcomings. The "Specialized Interceptors" were forced into a low altitude role where maneuverability was lacking but later on during the jet era became the "Frontline Interceptors". "Universal Fighters" also started out underpowered and slower but gradually became the best all-around frontline fighters and have gotten enhanced strike capability. "Universal Strike Aircraft" is basically the heavy attacker aircraft that's focused on ground support and later on advances in the shortcomings of speed. The "Specialized Strike Aircraft" are also generally heavily armed aircraft but are really the most unique prototypes  with the different primary role of air defense, escort and intercepting. Those later transition into faster, more practical interceptors. The "Strike Bombers" initially begin as light bombers and later on adapt to their shortcomings as more potent and maeuverable heavier attacker aircraft. The "Universal Bombers" are heavier strategic planes that focus the mostly on firepower but lack sufficient defensive armament and speed, later on adapts on those, while at the same time delivering more and more ordinance.

Anyway, without further ado, grab some snacks and enjoy the read. (Don't enjoy because not finished yet.)

Otherwise just to mention I decided to leave out the premiums the way they are. It was a little off-putting that some aircraft like the Tu-2 aren't on the standard or the Su-8 which gets unrealistically terrible handling, might negate this but I typically always found a way around it via a certain variant of these aircraft. Of course, like usual, you will see once you read the cards. By request I could try to insert them in (but I will probably leave out the foreign ones anyway).