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.
They can scout strategic areas for objectives or signs of enemy presence, and armies can move more quickly through areas which have been scouted by plane beforehand. No commercial distribution of images or text from any page on this site without written permission.