Fly any plane -
Hold [Ctrl] + [Alt], then select the "Quick Mission" option. Keep the keys 
pressed until the flight begins.

Atomic Moth -
Hold [Right Alt] + [Right Ctrl] + [Right Shift] while selecting any mission 
mode. The Atomic Moth will appear as an available aircraft. Any mission in 
campaign mode may also be selected if this is done when selecting "Play Single 
Mission" mode.