Your 2IC selects types of aircraft based on your strategy.
Your 2IC always tries to maintain 30% of your total aircraft as escort fighters and another 10% as reconnaissance aircraft.
As the strategy gets less cautious, he will favour more bombers at the expense of interceptors. (An "interceptor" is a fighter designed primarily for attack against enemy bombers, rather than the usually faster and more nimble enemy fighters. Interceptors often favoured firepower over manouverability compared to the pure fighter type).
For example, a very cautious strategy favours 30% as interceptors and 30% as bombers (of all types). A very agressive strategy favours 15% interceptors and 45% as bombers. Cautious and aggressive strategies lie in between.
Your 2IC strives to maintain these proportions in the face of losses. If all your losses were in fighter aircraft in the last turn, then construction this turn will predominantly be fighters, regardless of your strategy.
Note also that these proportions are based on aircraft primary roles. Most aircraft had two (or more roles), eg medium bomber and reconnaissance. But it is an aircraft's primary role that is always most important to your 2IC when selecting more aircraft to build.
Within a given type, your 2IC always tries to select the best available aircraft, which will usually be those more recently designed and available.
Because fighters are generally cheaper to build than medium and heavy bombers, a cautious or very cautious strategy will tend to "buy" more aircraft for the same expoenditure than an aggressive or very aggressive strategy. For example, it can take four times or more the RPs to build a heavy bomber than a small single-engined fighter.