Go to setup and check your keyboard map. There is a key in there for "select secondary weapon" (it's labeled something like that).
This is a toggle. If you have a plane with guns, rockets, and bombs, and you fly that plane, selecting this key (default is "backspace") will toggle your secondary guns to "guns" "rkts" and depending on the size of the bomb, that bomb's weight divided by 10 (500lb bomb would show up as "50") followed by the quantity for each.
The second bank, the one that usually says "CAN" is the one that will toggle. To get back to your guns after selecting bombs/rockets/whatever, select the secondary weapon select key until you have your guns back.