If you set the properties "flying" in the unit properties field it will start the game flying. If you want it to start a bombing run when certain conditions are met, place the plane very far away fromt he bombing location (better if it is on the sea) setting the "flying" property, then place a game logic in the location you want the plane starting from, and then moving it there using
bomber setPos(getPos bomberLogic)
You can name the bomber and the game logic as you please, just be sure you use the proper name with that command.
If you want it to follow a set of waypoints, place a move waypoint near the plane starting position, syncrhonize it with the trigger (remember that the trigger type must be "switch", otherwise it won't work) which moves the plane at the start of the bombing run, then place the set of waypoints you want it to follow. The switch type trigger basically makes the waypoint next the synchronized one be the current active waypoint when the trigger is fired. It is also useful to make a unit exit a loop waypoint sequence created with a CYCLE waypoint, or to make them exit a neverending waypoint, like HOLD or SEARCH AND DESTROY.