Hint: Shoot the parachutes down before they reach the ground and use machine guns
View the Aircraft In 3D
Remember all those Skilled Enemy Pilots that are marked as "DANGER!" on the Briefing Screen? If you manage to shoot every single one of them down and complete your collection of medals on the Statistics Screen, a new option will appear in the Options Menu. All the planes you have encounter so far (both yours and the enemies') may be now seen in 3D! You may rotate the cool planes around, and even zoom in and out!
Normal Ending
Play the game up to the 19th mission, "Kingpin". Finish the mission WITHOUT shooting down the skilled enemy pilot Z.O.E. (The enemy plane that is marked as 'DANGER' in the Briefing Screen.)
Music Test
If you clear the game with a rank higher than First Lieutenant, a music player mode will show up in the option menu. Here are the controls: press L1 or L2 to skip forward, R1 or R2 to skip backwards, Circle to select the current track, Select to toggle screen, Square or Triangle or X to end.
Bonus Aircraft
Finish the game and see the Normal or Bonus Ending. A new option will appear after you picked the difficulty under the "START GAME" option at the Title Screen. Using this option, you may have a chance to buy some new aircraft as you play through the game again.
Hidden Level
If you destroy the C-5 transport on the first raven level, a new mine level will become available, along with the regular one.