The idea is that these are early planes, used primarily for scouting and only minimally effective in direct combat. The pilots may be allowed a limited number of hand-bombs to fling over the side, but these are very inaccurate. Typically they hit the target point only on a doubles roll on 2D6. Draw a card or roll a die at once, and each movement turn thereafter, until the pilot secures the bomb, or the plane blows up. Aeroplanes are low and slow.
Any unit being bombed gets a chance to fire before the bomb is dropped.
Depending on your rules set, you can design reasonable modifiers or card draw combinations to make the planes vulnerable, but not sitting ducks. Aeroplanes can have other duties. One side may not know where its objective actually is until a plane has flown over the possibile sites and returned with a report. An aircraft might cause any native units which it flies directly over to take a morale check. Aeroplanes are probably most useful in campaigns.