If you mean practice mission #3, you have to bomb all 8 of the hangars, they are green or tan color. Best order in which to accomplish this is to take out enemy plane, take out enemy tanks, then bomb hangars at your leisure. You might miss a time or two and have to go all the way back to your base to land and reload bombs.
When I first started dogfight I got so pissed off trying to finish mission 3 that I don't know how my phone is still in one piece - its not like it TELLS YOU or anything.
It was the invisible zeppelin practice mission that was the icing on the cake. It caused me to back out of practice, click "multiplayer" for the first time, and I haven't gone back since