Actually, that is only partially true. There were no US Bombers, available to drop the extremely heavy and large bombs in the European theater. There was a British bomber that could have if the bombs had been developed sooner.
As it was, the bombing of Japan required two very specially modified Boeing B-29 Superfortress bombers that were modified and tested at the last minute.
“...As it was, the bombing of Japan required two very specially modified Boeing B-29 Superfortress bombers that were modified and tested at the last minute. [Robert357, post 111]
An incorrect statement.
Every B-29 assigned to 509CG was modified to carry and drop special weapons. They were modified in the USA, in some cases months earlier.