Dont forget that the Hiroshima bomb only had a couple hundred pounds of explosives.
Yes, I have heard of the Doolittle raid. That other than the B-29 was capable of delivering an atomic bomb is nothing I've heard before. Makes me wonder why we bothered to develop the B-29 and fight for a place in the Pacific to accommodate it bombing Japanese cities.
You haven't heard that because it isn't true. Only the B-29 were large enough to carry the early primative A bombs like fat man, and even then, the bombay doors had to be remade to drop the much larger load.