Robert K. Wilcox's book "Japan's Secret War" makes a VERY strong case that the Japanese were much farther ahead in the development of an atomic bomb then the Germans. There is zero question that the Japanese wanted and were willing to nuke US. At least we won that race.
I saw the History Channel, where they said the Japanese were planning a nuclear attack on the US, on Sept 17, 1945, though I am not sure if it was supposed to be a nuclear explosion, or simply a dirty bomb.