Weren't the bombs on the heavy buffs armed manually before drpping them?
Semp
They were, and several different strategies were employed. One system used on B17s (and others, I'm sure), was to remove a cotter pin which then allowed the "propeller" to spin, which armed the bomb. Once the pins were removed a long wire was threaded through the cotter pin holes, which prevented the fuses from arming from jostling in flight. When the bombs were dropped, they basically just un-threaded themselves from the wire as they fell the first few inches/feet.
Some bombs had cotter pins and fuses on the back of the bomb as well as the front. This was done as a measure to prevent "duds", which according to some accounts may have been up to 30% of the bombs dropped.
In some cases, the fuses were actually installed in flight. I've read that moist, humid conditions overnight, followed by the sub-freezing temps in flight rendered the fuses worthless, which then led to installing them in flight...