On the 3rd A-bomb, an excerpt:
The question often arises: did the United States have a third bomb ready to drop on Japan, following the Little Boy uranium device that destroyed Hiroshima on August 6 and the Fat Man plutonimium bomb that destroyed Nagasaki three days later?
In the Spaatz Papers at the Library of Congress manuscript section, there is much radio traffic generated on Tinian in the second week of August. The U.S. Army Strategic Air Forces wanted the third bomb to be dropped on Tokyo as a wakeup call for the Japanese government, which was stalling on agreeing to the United Nations surrender terms. (That this could have been seriously proposed is an indication of how woefully uninformed USASTAF was about the destructive power of the weapons it had delivered to the Empire.) Back came a message, presumably from Hap Arnold, saying that the decision had already been made that the target would be Sapporo in the northern island of Hokkaido.