it says in game. go to the your minisquadron page or w/e and click on the planes u dont have, and itll say what u have to do to unlock them
Don't have it in front of me, but doesn't it tell you how if you tap the individual planes? Edit: Ninja'd.
Most of them, require getting a certain score at a certain wave. Sometimes, they have two requirements. And as others have said, you can check in the game under "my squadron".
