The situation you describe and represented in the movie Dr. Strangelove nearly happened on March 11 1958 when Air Force Captain Bruce Kulka climbed into the bomb bay of a B-47 Stratojet because a fault light was triggered , upon inspection he accidentally hit an emergency release lever which dropped a Mark 6 nuclear bomb on Mars Bluff , North Carolina. The blast created 70'x50' wide and 30' deep crater near the home of Walter Gregg and his family. The blast was caused by 7,600lb of TNT that functioned as its triggering mechanism.

Thank you /u/DrSlappyPants for the addition - The fission core was not installed pryor to detonation. This prevented a nuclear disaster that would have been much worse.

Also worth mentioning nobody died from the explosion. Walter Gregg and his family of 6 survived the explosion with relatively minor injuries and were compensated 54,000 Dollars for the incedent by the U.S. Air Force.

Approximately equivelent to 435,000 Dollars in 2013 due to inflation.

To answer another question below it was the combined weight of Captian Kulka and the Mark 6 that forced the bomb bay doors open and in desperation the Captian managed to grab a piece of the B-47 and hoist himself up. The bomb was dropped from a height of 15,000 ft.

http://www.flocomuseum.org/artifacts/mars-bluff-bomb/

I will provide sources and expand when I get home as i'm on a mobile at the moment.

Could you describe the Dr. Strangelove case for those of us who haven't seen it?

I'm not sure if this is strictly relevant but here goes.

In "Big Week" by Glenn Infield, there's a story about an WW2 8th Air Force airman that was walking through the bomb-bay during a mission. A near-miss caused the plane to shake and the airman to fall off the catwalk. He ended up dangling out of the bomb-bay, suspended by his ankle, for several minutes until somebody managed to drag him back aboard. No, he wasn't wearing a parachute.