Actually, there was at least one incident that is pretty similar (although not identical) to that scene:
"The task was doomed from the start; later testimony indicated Kulka had no idea where to find the locking pin in the large and complicated bomb-release mechanism. After a tense 12 minutes searching for the pin, the bombardier decided, correctly, that it must be high up in the bomb bay and invisible because of the curvature of the bomb. A short man, he jumped to pull himself up to get a look at where he thought the locking pin should be. Unfortunately, he evidently chose the emergency bomb-release mechanism for his handhold. The weapon dropped from its shackle and rested momentarily on the closed bomb-bay doors with Captain Kulka splayed across it in the manner of Slim Pickens in Dr. Strangelove. Kulka grabbed at a bag that had providentially been stored in the bomb bay, while the more-than-three-ton bomb broke open the bomb-bay doors and fell earthward. The bag Kulka was holding came loose, and he found himself sliding after the bomb without his parachute. He managed to grab something-he wasn't sure what-and haul himself to safety. Moments later the plane was rocked by the shock wave of the blast when the bomb hit the ground."
"The task was doomed from the start; later testimony indicated Kulka had no idea where to find the locking pin in the large and complicated bomb-release mechanism. After a tense 12 minutes searching for the pin, the bombardier decided, correctly, that it must be high up in the bomb bay and invisible because of the curvature of the bomb. A short man, he jumped to pull himself up to get a look at where he thought the locking pin should be. Unfortunately, he evidently chose the emergency bomb-release mechanism for his handhold. The weapon dropped from its shackle and rested momentarily on the closed bomb-bay doors with Captain Kulka splayed across it in the manner of Slim Pickens in Dr. Strangelove. Kulka grabbed at a bag that had providentially been stored in the bomb bay, while the more-than-three-ton bomb broke open the bomb-bay doors and fell earthward. The bag Kulka was holding came loose, and he found himself sliding after the bomb without his parachute. He managed to grab something-he wasn't sure what-and haul himself to safety. Moments later the plane was rocked by the shock wave of the blast when the bomb hit the ground."
http://io9.com/5904633/in-1958-america-accidentally-dropped-...
NB Of course, only the chemical explosives in the bomb exploded.