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I can't imagine a WW2 crewman back in the bomb bay installing fuses, while the aircraft is flying and flak exploding all around, that just doesn't seem likely.
The American bombs i'm familiar with had a long stiff safety wire that went thru both the nose and tail fuse, and kept the propeller on the fuses from spinning.
If the bombardier threw the arming switch ( or the Pilot on a fighter) the arming wire stayed with the aircraft when the bomb fell away, freeing the propeller to spin. But the bomb wasn't actually armed until it fell far enough for the propeller to spin X number of turns. That usually rotated a block in the fuse between the firing pin and the initiating explosive.
If for some reason, the bombardier wanted to drop a bomb and not have it explode, he never tripped the arming switch, and the bomb fell with the arming wire still attached, and the propellers couldn't spin to arm the fuses.
IMO the bombs that went off when still in or on the aircraft didn't go off because something hit the bomb itself, the bombs were so thick, a bullet wouldn't penetrate most bomb casings, and even if it did, most high explosive is so stable even a bullet wouldn't faze it.
But the fuses are different, they've got a small amount of a fairly easy to set off explosive in them. If a bullet, or flak shrapnel, hits a fuse in the right area, it will explode.
Both bomb and fuse are mass produced, and usually by the lowest bidder.