The Manhattan Project employed 130,000 people at its peak, spanning physicists, military officers, engineers, factory workers, and industrial contractors spread across secret sites in New Mexico, Tennessee, and Washington state. While a handful of famous scientists tend to dominate the story, the project was a massive national effort involving Nobel laureates, Allied refugees, young women operating enrichment machinery, thousands of African American construction workers, and some of the largest corporations in America.
Military and Government Leadership
U.S. Army General Leslie Groves was appointed to lead the Manhattan Project in September 1942. He oversaw every major site: the plutonium production facilities at Hanford, Washington; the uranium enrichment plants at Oak Ridge, Tennessee; and the bomb design laboratory at Los Alamos, New Mexico. His immediate supervisor was Secretary of War Henry L. Stimson, who authorized project sites and ensured the effort received whatever resources it needed.
General Kenneth Nichols served directly under Groves as Deputy District Engineer and later District Engineer of the Manhattan Engineer District, with direct responsibility for both Oak Ridge and Hanford. Franklin Matthias, a civil engineer, selected Hanford as the plutonium production site and supervised its construction. Above them all in the scientific chain of command sat Vannevar Bush, who chaired the National Defense Research Committee starting in 1940 and later directed the Office of Scientific Research and Development. Hans Bethe called Bush the leader of American scientific work in support of the entire Allied war effort.
J. Robert Oppenheimer and the Los Alamos Scientists
J. Robert Oppenheimer directed the Los Alamos weapons laboratory, where the actual bomb designs came together. Under him worked a remarkable concentration of talent. Hans Bethe led the theoretical division. Enrico Fermi, who had already won the 1938 Nobel Prize in Physics, headed the F Division and was present at the Trinity Test in July 1945. Emilio Segrè, who had helped discover plutonium-239, served as a group leader. Luis Alvarez, a future Nobel laureate, worked on blast wave predictions.
The lab also drew heavily on European refugees. Leo Szilard, a Hungarian physicist, had been instrumental in convincing Albert Einstein to write the famous 1939 letter to President Roosevelt warning about German atomic research. Eugene Wigner, another Hungarian émigré, contributed critical theoretical work. Edward Teller, also from Hungary, pushed early concepts for a thermonuclear weapon during the project.
Nobel Laureates Across the Project
The Manhattan Project gathered an extraordinary number of Nobel Prize winners, past and future. Arthur Compton, who had won the 1927 Nobel Prize in Physics, ran the Metallurgical Laboratory in Chicago, overseeing Fermi’s creation of the first nuclear reactor and the operation of the X-10 Graphite Reactor at Oak Ridge. Ernest Lawrence, the 1939 Nobel laureate in Physics for inventing the cyclotron particle accelerator, developed the electromagnetic method of separating uranium isotopes.
Glenn Seaborg joined the Metallurgical Laboratory in 1942, where he figured out the multi-stage chemical process to separate and concentrate plutonium from uranium. He later won the 1951 Nobel Prize in Chemistry. Harold Urey, who had already won the 1934 Nobel for discovering deuterium, led isotope enrichment work at Columbia University. He developed the gaseous diffusion process that became the primary uranium enrichment method after the war.
The First Nuclear Reactor Team
On December 2, 1942, forty-nine scientists stood in a converted squash court beneath the University of Chicago’s abandoned Stagg Field and watched the world’s first nuclear reactor go critical. Fermi led the experiment. Among those present were Compton, Szilard, Wigner, Leona Woods (the only woman in attendance), Herbert Anderson, Walter Zinn, Louis Slotin, and George Weil, who manually controlled the reactor’s cadmium control rod. This moment proved that a self-sustaining nuclear chain reaction was possible, and it gave the project its technical foundation.
The British Mission
The Manhattan Project was not purely American. A group of European and British scientists known as the British Mission worked alongside American teams, particularly at Los Alamos. The group included Nobel laureate James Chadwick, who had discovered the neutron, and Niels Bohr, the Danish physicist who escaped Nazi-occupied Europe with his son Aage. Future Nobel laureate Joseph Rotblat from Poland also participated.
These scientists filled specific technical gaps. James Tuck helped develop the shaped explosive lenses needed for the implosion-type bomb. Rudolf Peierls, a German refugee, contributed numerical solutions to blast wave problems. Otto Frisch, an Austrian émigré, led the Critical Assemblies group, doing the dangerous work of testing how close fissile material could get to a critical mass. William Penney calculated the optimal detonation altitude for the bombs dropped on Japan and predicted blast damage effects. After the war, Penney went on to lead Britain’s own nuclear weapons program.
One member of the British Mission, Klaus Fuchs, was later confirmed to be a Soviet spy who passed secret nuclear information to Russia throughout his time on the project.
Industrial Contractors and Corporate Partners
The sheer scale of the Manhattan Project required major American corporations to build and operate its facilities. DuPont designed and built the X-10 reactor at Oak Ridge and designed, built, and operated the plutonium production reactors at Hanford. Stone and Webster Engineering designed and built the electromagnetic enrichment facilities at Oak Ridge, which were then run by Tennessee Eastman Corporation. M.W. Kellogg Company built the gaseous diffusion plant at Oak Ridge, operated by Union Carbide.
On the academic side, the University of Chicago administered the Metallurgical Laboratory and the University of California administered Los Alamos. The U.S. Army Corps of Engineers employed several thousand technically trained individuals across all sites.
The Calutron Girls and Women’s Roles
Starting in 1944, young women at Oak Ridge’s Y-12 plant operated the calutrons, massive machines that separated the lighter uranium-235 (the fissile isotope needed for a bomb) from the heavier, more common uranium-238. Known as the Calutron Girls, these women monitored dials and kept the enrichment process running smoothly. They actually outperformed the scientists at the task. Because they didn’t know the underlying physics, they were more likely to flag a problem for a supervisor rather than try to fix it themselves, and their adjustments on the dials proved more precise than those of scientists who tended to overthink the process.
Leona Woods was the only woman present at the first nuclear reactor demonstration in Chicago. Maria Goeppert Mayer, who later won the Nobel Prize in Physics, also contributed to the project’s scientific work.
African American Workers
African Americans made up a significant portion of the Manhattan Project’s workforce, though they faced segregation and limited roles. At Hanford, DuPont extensively recruited Black workers between 1943 and 1945. Approximately 15,000 African Americans traveled to the Tri-Cities area in Washington state, many leaving families in the South. Out of roughly 50,000 workers at Hanford in July 1944, about 5,000, or 10 percent, were African American. Army officials deliberately capped that percentage at 10 to 20 percent of total employees.
Most African Americans at Oak Ridge and Hanford worked as construction laborers, janitors, and domestic workers. Their labor was essential to building the facilities that produced the plutonium for the “Fat Man” bomb dropped on Nagasaki. Some Black workers held scientific and technical positions in Chicago and New York, but these were the exception. Housing was segregated, facilities were separate, and living conditions for Black workers were consistently worse than those provided to white employees.
The Enlisted Soldiers With Science Degrees
Beyond the famous names, the project relied on hundreds of young soldiers with technical training. The Special Engineer Detachment, or SED, was composed of enlisted men with scientific backgrounds. About 29 percent held college degrees, and most were mechanical, electrical, or chemical engineers. By the end of 1943, nearly 475 SED members had arrived at Los Alamos alone. A companion unit, the Provisional Engineer Detachment, handled additional support tasks. These soldiers did hands-on laboratory and engineering work that kept the research moving day to day.
In total, the Manhattan Project spent $2.2 billion by the end of the war (roughly $37 billion in today’s dollars) and connected a vast, secret network of physicists, soldiers, factory operators, construction crews, and corporate engineers, most of whom had no idea what they were building until the bombs fell on Hiroshima and Nagasaki in August 1945.