The crews of the 509th had trained together for almost a year under top-secret conditions. They had first gathered at Wendover Field, an isolated base in western Utah, and then had flown individual long-range, over-water navigation missions from Batista Field, Cuba. The personnel of the 509th moved to Tinian by air and sea in late May and early June 1945, where their top-secret status was the subject of much curiosity and constant ribbing. The crews designated for the atomic missions practiced by dropping giant 10,000-pound “pumpkins” on 12 Japanese targets. Each pumpkin contained 5,500 pounds of explosives.

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The B-29s of the 509th had been modified to deliver the atomic bomb and were thus unable to carry conventional bombs. Instead, they carried the pumpkins, painted orange and shaped like Fat Man. The pumpkins also had been used during their Stateside training. Proximity fuses that produced an air burst, a feature of the atomic bombs, were installed. About 45 of the pumpkin bombs had been brought from the States. According to Tibbets, his crews were so accurate with them that Maj. Gen. Curtis E. LeMay, then commanding the Twentieth Air Force, ordered 100 more.

The carefully planned elements of one of the world’s most singular air units came together on schedule, backed by the highest national priority for supplies. The two atomic bombs were the result of the work of thousands of people. They had accepted the responsibility to try to split the atom, and to explore its potential as a bomb that could be controlled and released on demand.

The development of the atomic bomb can be said to have begun in the 1920s and early 1930s. It was then that several physicists, most of them in Europe, originated theories about ways to unlock the energy they believed existed within the atom. One of those physicists was Leo Szilard, a Hungarian who had fled from Nazi Germany to England in 1933. Szilard theorized that “in certain circumstances, it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale and construct atomic bombs.” He urged British officials to conduct research to prove or disprove his theory.

Meanwhile, two German physicists, Otto Hahn and Lise Meitner, experimented with radioactive uranium in an effort to produce a chain reaction. Meitner fled from Nazi Germany to Sweden in 1938 and, together with Otto Frisch, passed the results of their experiments to physicist Niels Bohr, who left soon after for the United States. Bohr contacted Albert Einstein, also a refugee scientist, and winner of the 1921 Nobel Prize for physics, to explain the military potential of atomic energy.

Einstein, by then well-known in America, wrote a letter in August 1939 to President Franklin D. Roosevelt. “Some recent work,” his letter said, “…leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future and it is conceivable…that extremely powerful bombs of a new type may thus be constructed.”

Roosevelt appointed a group of scientists to an advisory committee on uranium, but at the time there was no real stimulus to proceed with any definitive action. Meanwhile, scientists in Germany and Japan were also considering the potential of atomic energy for war use. It took the attack on Pearl Harbor to stir the United States into action.

In 1942, Dr. Vannevar Bush, head of the U.S. Office of Scientific Research and Development, confirmed to the president that an atomic weapon could be developed. The Manhattan Project was authorized. General Leslie R. Groves, a tough, no-nonsense Army Corps of Engineers officer, was put in charge.

Enrico Fermi, an Italian physicist working with a team of fellow scientists at the University of Chicago, built the first nuclear reactor on a squash court under the stands of the university’s football stadium. On December 2, 1942, the world’s first self-sustaining, controlled nuclear reaction was achieved.

There were at least two methods that could be used to produce an explosion, both expensive but possible. Extensive facilities were built at Oak Ridge, Tenn., and Hanford, Wash., to produce uranium and plutonium, the fissionable material needed for the bombs. A central laboratory to design both bombs was established at the so-called Site Y near Los Alamos, N.M., with Dr. J. Robert Oppenheimer in charge.

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