Not really. They had very high confidence that the weapon would destroy itself essentially no matter what. The only circumstance in which they thought this might not be the case is if it was dropped into water (in which case it would basically become a nuclear reactor and be difficult to recover). The bomb was so crude, and so simple, that it would have likely had a nuclear yield in almost any plausible scenario. If it was dropped from 30,000 feet, it would be impacting the ground at some 1,100 feet per second — definitely enough force to cause the two pieces of the uranium to smash into each other.

One can imagine what they might have done if this proved wrong (launch a massive incendiary raid on the city? try to rush the next bomb?) but I have never seen any documents that discuss it. They don't seem to have thought it was likely-enough to pre-plan for.