You know if I was wanting to build this I think you could make a simple setup where you had a couple pins to locate the bomb, and use a servo with a strong magnet mounted in the wing or belly of the plane. Then a magnet in the bomb would hold it tight .. when the servo pulled the magnet in the plane back from the bomb it would fall. I think it could be a simple but effective way to do it...
Just an idea if you like to experiment.