To unlock the X-wing and Tie Fighter in Instant Action Mode, beat all 15 missions and all 22 challenge missions.
Invinicibility
At the main screen that says "Continue" and "New Game" press Up, Down, Left, Right, Left, Left, Right, Right, L1, L1, R1, R1, L2, R2. If you do it correctly, the game will say "Secret Code Accepted".
Unlimited Ammo
At the main title screen that says "Continue" and "New GAme" press Up, Right, Down, Left, Up, Right, Down, Left, L1,R1. If you do it correctly, the game will say "Secret Code Accepted".
Unlock All Levels
At the new game/continue screen press Up, Down, Left, Right, L1, R1, L1, R1. This will unlock all levels in Instant Action Mode.
Big Heads In Cockpit
At the new game/continue screen press Up, Right, Up, Left, Down, Right, Up, Left, Down, Right, ,L1, R1, L1, R1.
XP-55
Successfully complete the bonus objective of sinking all patrol boats in "Mosquito Bite" to unlock the Curtiss-Wright XP-55.
Get All Medals
You have to beat all of the secondary objectives. The catch is you have to beat them on the first try.
