If you are using a joystick or gamepad the best soultion would be to program a button to release the selected weapon. In order for you to release all of your bombs in a short period of time* just increase the repetition rate to full. This will act just the same as a turbo button, so holding it down will repeat process over and over again until your bomb bay is empty.

*In real life bombers cannot drop all of their bomb load at the same time to cause a widespread area of damage**. Instead they are released in a sequential order.

**This is due to the inaccuracy of bombers in WWII. Bomber formation did not have to target small targets of opportunity. They are they primarily used to carpet an area with as many explosives as possible like cities and production centers.