Also, the flight refuelling thing wasn't ready by August 1945; trials were still undergoing.
It would have been eventually though:
Flight refueling is undoubtedly a successful method of increasing the range and or bomb load of an aircraft, and the number of unsuccessful sorties during the trials was only two out of 36 flights. While single aircraft could be refueled without difficulty, the question of refueling large numbers of aircraft in a short space of time becomes more complicated.
- Bomb Development Unit
An interesting side note is that after the bombs were dropped and everything was cancelled, some of the stored equipment was later refurbished, modified, and delivered to the US to meet an urgent requirement for flight refueling SAC B-29s.