Fat Man and Little Boy were armed by people crawling over them in the bomb bay to do various things (including inserting fuses iirc).
More modern bombs were/are armed electronically by the weapon engineer or bombardier.
In the crash cases the regular explosive in the bombs went off in a few cases, but because the bombs weren't armed all it did was scatter casing and nuclear material over a wide area, rather than nuking the east coast of the USA or Southern Spain or a bit of ocean etc.
The one in Spain was a bit of an embarrassment, as the US had to pay to resettle people and haul away millions of tons of topsoil for burial in the US.
Biting the hand that feeds IT
