Spitfires are Air Superiortiy Fighters. They only have droptanks (Except the Spitxteen) and even if they had more, it would violate the Spitfires name to use them.
Fly american planes, they have a lot of extra weapons. Like he said, select in hangar. In air, press backspace to select which secondary weapon you want to use, and press your secondary fire button. The default for that is, I beleive, B (The first thing I did was map secondary fire to my joystick)