Greets nopoop,
<backspace> should cycle selection between your secondary weapons. The selected weapon should show up on the cockpit display for the secondary weapon (could show gallons remaining in your drop tanks, #cannon rounds remaining, or the size of the bombs you've selected). these are all considered secondary weapons - Drop tanks do explode when dropped but I don't know if they do any damage...
Atta boy, keep up the fight! If I can't find a fight, I often sling a bomb under a P-39Q and head to a field to take out some targets - often my presence will entice some uppers....