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. (I read this material while researching a magazine article in 1995.)
In Bulletin of the Atomic Scientists, May-June 1998, Stanley Goldberg notes that on the morning of August 10, 1945, Robert Bacher of the Environmental Physics Division of Los Alamos National Laboratory was supervising the loading of a plutonium core onto a truck. The core (presumably the casing and other "works" were already on Tinian or en route) was to be flown to San Francisco, thence to Tinian, to finish its journey over the city of Kokura about August 20. Robert Oppenheimer then appeared and told Balcher to stop loading the core. No further shipments were to be made, Oppenheimer said, without an explicit order from President Truman.
(In what seems to me to be a logical leap, Goldberg concludes: "Since Truman could have given such an order at any time between July 24 and August 9, it strongly suggests that the bombing of Nagasaki came as a surprise to him." Goldberg suggests that Major General Leslie Groves alone had directed the bomb's use on Nagasaki, as a bureaucrat anxious to justify the money that gone gone into its development, and also as a military man who wanted to hasten the end of the war.)
Al Christman's book, Target Hiroshima: Deak Parsons and the Creation of the Atomic Bomb (Naval Institute 1998), notes that the operational plan in February 1945 "called for the military use in the summer [of 1945] of Little Boy and one or two Fat Man bombs, followed by more if necessary." In July, following the Trinity test of the plutonium bomb, General Groves remarked: "The war is over as soon as we drop two of these on Japan." The cruiser Indianapolis brought Little Boy to Tinian on July 26; Christman makes no mention of Fat Man. On July 28 and 29, four "Green Hornet" transports flew in from the U.S. with the plutonium pieces for Fat Man and the uranium inserts for Little Boy.
Elsewhere, Christman notes that "Parsons had planned and organized the Tinian assembly facilities to handle a steady stream of bombs [after Little Boy devastated Hiroshima]. The plutonium production facilities at Hanford continued to work at capacity ... everything needed for the second bomb was present at Tinian, and essential materials for a third bomb would soon be on their way." When the B-29 stand-down went into place, Parsons was about to go home, but Groves stopped him "in order to assure complete readiness to assemble and deliver additional atomic bombs in the event that negotiations with the Japanese broke down."
Charles Sweeney published his memoirs as War's End: An Eyewitness Account of America's Last Atomic Mission (Avon, 1997). During the party following the successful Hiroshima drop, he recalled that Paul Tibbets took him aside and told him that he was to command the second atomic mission, with Kokura as the primary and Nagasaki as the secondary target. Timing was important, Tibbets said: "It was vital that [the Japanese] believed we had an unlimited supply of atomic bombs and that we would continue to use them. Of course, the truth was that we only had one more bomb on Tinian. Delivery of the third bomb was several weeks away."
Major Sweeney flew one of eight 509th Composite Group B-29s that took part in the war's final mission, the "thousand-plane raid" of August 14-15. Enola Gay and Bock's Car were excused "for obvious reasons," as was The Great Artiste, which because it contained the scientific instruments that would be needed if there were a third atomic mission. The group's two remaining B-29s, he noted, were Spook and Jabett III--and they "were on route to the United States to take delivery of components for more Fat Man bombs."
In an August 2002 interview with Studs Terkel published in the British Guardian newspaper, Paul Tibbetts recalled something similar: "Unknown to anybody else--I knew it, but nobody else knew--there was a third one. See, the first bomb went off and they didn't hear anything out of the Japanese for two or three days. The second bomb was dropped and again they were silent for another couple of days. Then I got a phone call from General Curtis LeMay. He said, 'You got another one of those damn things?' I said, 'Yessir.' He said, 'Where is it?' I said, 'Over in Utah.' He said, 'Get it out here. You and your crew are going to fly it.' I said, 'Yessir.' I sent word back and the crew loaded it on an airplane and we headed back to bring it right on out to Trinian and when they got it to California debarkation point, the war was over."
In Downfall: The End of the Imperial Japanese Empire, Richard Frank says it was General Marshall and General Grove who delayed the transport of the third bomb, sufficient that it couldn't have been deployed until August 21 or thereabouts.
Chuck Hansen's great book U.S. Nuclear Weapons: The Secret History doesn't explicitly go into this question, but it does note that at the end of 1945 the U.S. owned a total of two atomic bombs, both Fat Man plutonium bombs. (This design became the standard U.S. nuclear weapon until into the 1950s.) He also notes that the weapons were short-lived, so it is possible that a) there were more than two bombs in the inventory when the war ended and even that b) the bombs on hand on December 31 had been assembled after August 15.
TimelineThere's a timeline at www.ask.ne.jp/~hankaku/english/np7y.html. Here are some of the highlights touching on the "third bomb":
May 10: 2nd Target Committee meeting priotizes targets as 1) Kyoto, 2) Hiroshima, 3) Yokohama, and 4) Kokura
May 30: Kyoto removed from target list by order of Henry Stimson, secretary of war
June 10: 509th Composite Group arrives on Tinian, the Marshall Islands, with 11 B-29s
July 16 (four hours after the Trinity explosion): Little Boy shipped aboard USS Indianapolis
July 23: second plutonium core completed (unclear whether this means the core for the Nagasaki bomb or for the third bomb)
July 26: Indianapolis arrives at Tinian; Little Boy unloaded
Same day: uranium warhead for Little Boy (the Hiroshima bomb) sent to Tinian by C-54 transport plane
Same day: plutonium core and initiator for Fat Man (the Nagasaki bomb) sent to Tinian by C-54
August 2: "Parts of Fat Man arrive at Tinian"
August 11: "Interruption of transport to Tinian of the 2nd Plutonium core and initiator by the order of G.C. Marshall"
There are gaps here, but it seems pretty obvious that on August 11 the heavy stuff for the third bomb (Fat Man #2) was either already on Tinian or soon to arrive by sea, and that it could have been married up with the fissile core within a week.