Now I get it! Thanks for the explanation. It's hard to tell exactly how to do that without the capx. But, at first, using the bomb with bullet behavior and Set angle to Yes should work fine, unless you are using a global gravity or something like it. In that case, you could controll gravity according to the angle of the plane and it would work fine too.
If you could share the capx, I could give you a more specific answer, though.