The following codes are used at the main menu screen:

Get $400 Million:
R2, L2, R2, L1, R2, R1, R2, L1

Bonus Airplanes in N.GEN Mode:
R1, R2, L1, L2, L2, L1, R2, R1

Unlock All Tracks and Aircraft in Arcade Mode:
R1, L1, R1, R2, L2, R2, L2, L1

Secret Jet:
Beat both circuits of any type in first place to unlock a secret jet.

Thanks to Revolution reader Jodacky!