By 1944 the Manhattan Project was spending money at a rate of more than $1 billion per year. The situation was likened to a horse race—no one could say which of the horses (the calutron plant, the diffusion plant, or the plutonium reactors) was likely to win or whether any of them would even finish the race. In July 1944 the first Y-12 calutrons had been running for three months but were operating at less than 50 percent efficiency; the main problem was in recovering the large amounts of material that splattered throughout the innards of the calutron without reaching the uranium-235 or uranium-238 receiver bins. The gaseous diffusion plant, known as K-25, was far from completion, with the production of satisfactory barriers remaining the major problem. And the first plutonium reactor at Hanford had been turned on in September, but it had promptly turned itself off. Solving this problem, which proved to be caused by absorption of neutrons by one of the fission products, took several months. These delays meant almost certainly that the war in Europe would be over before the weapon could be ready. The ultimate target was slowly changing from Germany to Japan.

Within 24 hours of Roosevelt’s death on April 12, 1945, Pres. Harry S. Truman was told briefly about the atomic bomb by Secretary of War Henry L. Stimson. On April 25 Stimson, with Groves’s assistance, gave Truman a more extensive briefing on the status of the project: the uranium-235 gun design had been finalized, but a sufficient quantity of uranium-235 would not be accumulated until about August 1. Enough plutonium-239 would be available for an implosion assembly to be tested in early July; a second would be ready in August. Several dozen B-29 bombers had been modified to carry the weapons, and construction of a staging base was under way at Tinian, in the Mariana Islands, 2,400 km (1,500 miles) south of Japan.

The test of the plutonium weapon was named Trinity; it was fired at 5:29:45 am on July 16, 1945, at the Alamogordo Bombing Range in south-central New Mexico. The theorists’ predictions of the energy release, or yield, of the device ranged from the equivalent of less than 1,000 tons of TNT to the equivalent of 45,000 tons (that is, from 1 to 45 kilotons of TNT). The test actually produced a yield of about 21,000 tons.

The weapons are used

1 of 3

2 of 3

3 of 3

A single B-29 bomber named Enola Gay flew over Hiroshima, Japan, on Monday, August 6, 1945, at 8:15 am. The untested uranium-235 gun-assembly bomb, nicknamed Little Boy, was airburst 580 meters (1,900 feet) above the city to maximize destruction; it was later estimated to yield 15 kilotons. Two-thirds of the city area was destroyed. The population present at the time was estimated at 350,000; of these, 140,000 died by the end of the year. The second weapon, a duplicate of the plutonium-239 implosion assembly tested in Trinity and nicknamed Fat Man, was to be dropped on Kokura on August 11; a third was being prepared in the United States for possible use 7 to 10 days later. To avoid bad weather, the schedule for Fat Man was moved up two days to August 9. A B-29 named Bockscar spent 45 minutes over Kokura without sighting its aim point. The air crew then proceeded to the secondary target of Nagasaki, where at 11:02 am the weapon was airburst at 500 meters (1,650 feet); it was later estimated that the explosion yielded 21 kilotons. About half of Nagasaki was destroyed, and about 70,000 of some 270,000 people present at the time of the blast died by the end of the year.