It's action that counts, not words...
Just to make this easy we'll call Fat Man a 15KT device. 15,000 tons is 30,000,000 pounds. Once B-29 can carry 20,000 pounds, so 15KT is roughly equal to 1,500 B-29 loads. There are three B-29s in a formation, so a 15KT device would only take 500 sorties.
...if you start now you will just about have enough time to haul the whole thing before the resolutions are passed and the clever speeches end...