Weapon 1 default is generally MGs
Weapon 2 default is Cannons or secondary MGs
Weapon 3 default is Rockets
Weapon 4 default is Bombs
You can also map a key for Weapons 1 & 2 to fire simultaneously
Drop tanks can be bound separately to a further key (or key-combo) in the Controls section.
However, you cannot map the SAME key to two operations using the Control set-up in the game. Also you cannot map drop-tanks to a Weapon release command. You CAN map two keystrokes to a single command - or one keystroke and a joystick button - so bomb release could be on the keyboard AND the joystick, or it could be both Key A and Key B (but that would be wasteful of precious keystrokes). But you cannot make the Jettison-tanks command drop the bombs or vice versa.