I have done this many times. Be sure to be over 1o00 feet above the enemy plane or set of bombers. This allows for the 800 feet to activate the bomb. Be sure to be slightly ahead of your target plane. Letting all bombs will help in the overall effect. Flying a set of bombers with little bombs is the best.
I have also used the pilot controlled front gun in B-26 formation to dive on another set of enemy bombers and eliminate all 3 bombers in one dive pass from the rear.
This sandbox game allows a player to do things not normally done in real life. That's where the smiles come in.