What if the bomb had not detonated at Hiroshima?

Watching a recent doentary on the nuclear bombing of Hiroshima, i got to thinging, what if the bomb had failed to work. the doentary said that the bomb operator on the Enola Gay, William Parsons, embled the triggering device in flight. If he had done it wrong (if) and the bomb failed to detonate and just dropped, and did not explode, into a Hiroshima street?