This photo originated from the estate of John Frederick Lemp, Sr. (1894-1977) the son of John Emil and Caroline Emilie (Zeltman) Lemp of Boise, Idaho. 

In April 1942 Edward Mallinckrodt was approached by a contingent from the Metallurgical Laboratory of the Manhattan Project, particularly by Arthur Holly Compton. Compton urgently needed a source of refined uranium. The chemical company already worked with ether, which could theoretically be used in a purifying process but which posed its own handling risks. With extreme haste and with tight security restrictions, Mallinckrodt developed a novel technique from theoretical concept, to experiment, to full production. The company submitted test materials by mid-May, supplied the material for the first self-sustaining reaction in December, and had satisfied the project's entire order of the first sixty tons before the contract with the government was even signed. From 1942 to 1957, Mallinckrodt processed uranium ore into weapons-grade uranium at a factory north of downtown St. Louis.

The discovery of nuclear fission in 1938 made the development of an atomic bomb a theoretical possibility. Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein–Szilard letter to Roosevelt in 1939. This prompted preliminary research in the United States in late 1939. Progress was slow until the arrival of the British MAUD Committee report in late 1941, which indicated that only 5 to 10 kilograms of isotopically-pure uranium-235 were needed for a bomb instead of tons of natural uranium and a neutron moderator like heavy water. Consequently, the work was accelerated, first as a pilot program, and finally in the agreement by Roosevelt to turn the work over the U.S. Army Corps of Engineers to construct the production facilities necessary to produce uranium-235 and plutonium-239. This work was consolidated within the newly created Manhattan Engineer District, which became better known as the Manhattan Project, eventually under the direction of Major General Leslie R. Groves, Jr. The work of the Manhattan Project took place at dozens of sites across the United States, and even some outside of its borders. It would ultimately cost over US$2 billion and employ over 125,000 people simultaneously at its peak. Groves appointed J. Robert Oppenheimer to organize and head the project's Los Alamos Laboratory in New Mexico, where bomb design work was carried out. Two different types of bombs were eventually developed: a gun-type fission weapon that used uranium-235, called Little Boy, and a more complex implosion-type nuclear weapon that used plutonium-239, called Fat Man.

The Little Boy A-Bomb killed between 90,000 and 146,000 people in Hiroshima which was selected as a target because it was considered an important army depot and port of embarkation in the middle of an urban industrial area.

During World War II, Arthur Compton was a key figure in the Manhattan Project that developed the first nuclear weapons. His reports were important in launching the project. In 1942, he became head of the Metallurgical Laboratory, with responsibility for producing nuclear reactors to convert uranium into plutonium, finding ways to separate the plutonium from the uranium and to design an atomic bomb. Compton oversaw Enrico Fermi's creation of Chicago Pile-1, the first nuclear reactor, which went critical on December 2, 1942. The Metallurgical Laboratory was also responsible for the design and operation of the X-10 Graphite Reactor at Oak Ridge, Tennessee. Plutonium began being produced in the Hanford Site reactors in 1945. After the war, Compton became chancellor of Washington University in St. Louis.