In order for the B-29 to be used in Europe, the US would have had to invest in upgrading the bomber airfields the planes would be operating from. That would have taken a considerable effort and stripped construction materials from things like building the Mullberry harbors for the Normandy invasion. While it wouldn't have taken a majority, it still would have hurt that production effort. Then there was little advantage to using the B-29 in Europe. Its range would have been wasted, that being its main advantage. As for atomic weapons, three bases in the UK were chosen by the US for upgrading to use B-29's regardless of whether they'd be deployed or not. All three had loading pits for nuclear weapons installed as part of those upgrades. So, if the US needed to nuke Germany, the necessary basing and support resources were available in England and a B-29 or 29s could have been sent there to nuke Germany. Mildenhall, Marham, and one other I can't recall were converted. Mildenhall supported B-29's at the end of the war, and shortly afterwards as part of Project Ruby. The RAF got the B-29 postwar as the Washington B1 bomber and it was stationed at a number of RAF bases that were upgraded to handle the plane.