Ok so if from the main menu if you select Aircraft, you will notice there are 6 alternatives greyed out.
If you click on these it will tell you they are possible loot drops from the aces.
I didn’t get one when I first beat the aces, but replaying 1-5 again and killing the ace for the second time, he dropped an aircraft.
As such, to unlock 5, you will need to destroy each of the aces (well 5) a second time, after you have initially completed the level.
Good luck!!