I think that there are certain things that each planeset to have and I think that this should be a basis of what each counry needs and some planes I recommend for some of these postions. Also some planes can run over and cover each period or roll and also some countries may not have an applicable plane( such as the German's not having a heavy bomber). The American planeset shall be my example of a fleshed out set. Also everyone feel free to change my set or make your own
2 or more fighters P-38 p-47 P-51
at least 1 fighter with attack capabilities b-38 p-47 p-51
a pure attack aircraft a-20
light bomber b-25, boston 3
medium bomber b-26
heavy bomber b-17, b-24
It would also be important to try to fill in as many rolls as possible for each time period although not all countries would be able fill in the different areas the earlier you go.