I have come across a reference to early nuclear weapons when they had predicted problems with the B29 design. It actually has 2 bomb bays with the wing spar between them, in action bombs being dropped simultaeniously from front and rear bays. Dropping one very large bomb would require cutting through the main spar and rearranging the bay. The alternative was to use an existing bomber with a big enough bomb bay; the only one available being the Lancaster. Further tests however proved the B29 'silver plate' modification being suitable for the mission. This not only involved altering the bomb bay but also removing all possible excess weight, including all defensive armament. As a standard Lancaster would not have had the range required, in-flight refueling would have been essential.