The B-17 was used for a significant portion of the Allied campaign during WWII. My understanding is the earliest bombs in aviation consisted of just throwing stuff out of the open cockpit of planes. Later this progressed to some doors in the fuselage through which bombs simply fell out when opened.
The B-17 appears to have a more complex bomb bay system, with doors and with the bombs mechanically retained somehow. Did Allied crews ever experience difficulty getting the bombs to release? How did they deal with this (landing with bombs seems dangerous)? Does any data exist for what percentage of missions encountered problems with the bomb release mechanisms?