The Manhattan Project started years before that, in response to Albert Einstein's letter to FDR about the German atomic efforts.
The B-36 design was started when we were not sure the Germans would not invade/capture England.
We were not going to be flying bombers 5000 miles to drop a few tons of TNT and fly 5000 miles back.
The program was started when the prospect of German independent development of atomic weapons was a serious possibility. We could have been treated to the spectre of an atomic weapon successfully dropped on America by Germany. The US having atomic weapons would forestall this from ever happening as a deterrent.
That was the fear in ‘39.