https://en.wikipedia.org/wiki/Third_Shot

The Third Shot was an American nuclear weapon intended for use against Japan in World War II, subsequent to the nuclear attacks on Hiroshima and Nagasaki. It was intended to be used on 19 August 1945, ten days after the bombing of Nagasaki. It was the first in a series of additional atomic bombs being assembled to strike Japan but was never used, as the country’s surrender on 15 August brought the war to a close. The Third Shot was a plutonium-239-based implosion bomb of the “Fat Man” design, similar to the bomb that was dropped on Nagasaki.