The atomic bomb was built by the Manhattan Project, a secret American-led military effort that at its peak employed nearly 129,000 people across dozens of sites in the United States, Canada, and the United Kingdom. No single person built it. The bomb was the product of a massive collaboration between military leaders, theoretical physicists, industrial corporations, and tens of thousands of ordinary workers, many of whom had no idea what they were building. The project cost nearly $2 billion by the end of 1945 (roughly $22 billion in 1996 dollars) and culminated in the first nuclear detonation at 5:30 a.m. on July 16, 1945, in the New Mexico desert.
The Two Men Who Led the Project
The Manhattan Project was run by an unlikely partnership: Army General Leslie Groves and theoretical physicist J. Robert Oppenheimer. Groves, appointed commanding general of the Manhattan Engineer District in 1942, was brusque, plainspoken, and a relentless decision-maker with little patience for diplomacy. He oversaw the entire operation, from selecting production sites to managing billions in funding to maintaining extreme secrecy across a workforce the size of a small city.
His most consequential decision was choosing Oppenheimer to lead the weapons laboratory at Los Alamos, New Mexico. It was a controversial pick. Oppenheimer had never managed a large project, and his past political associations raised security concerns. But Groves saw something in him. Oppenheimer turned out to be a brilliant organizer who could speak the language of dozens of different scientific disciplines and keep a sprawling team of world-class physicists moving toward a single goal. The two men were opposites in temperament, which caused friction, but they formed what the Department of Energy’s historical records describe as “a very effective, if highly unlikely, team.”
The Scientists at Los Alamos
Los Alamos, the secret laboratory perched on a mesa in northern New Mexico, was where the bomb itself was designed and assembled. Oppenheimer recruited an extraordinary concentration of scientific talent. American-born physicists like Arthur Compton, Glenn Seaborg, Ernest Lawrence, and Robert Serber became central figures. Enrico Fermi, the Italian-born physicist who had already achieved the first controlled nuclear chain reaction in Chicago in 1942, was a key contributor. Richard Feynman, then in his mid-twenties, worked on calculations critical to the bomb’s design and went on to become one of the most celebrated physicists of the twentieth century.
A significant contingent came from Britain. Known as the British Mission, this group included some of Europe’s finest experimental and theoretical physicists along with specialists in electronics and explosives. Their contributions were far from peripheral. Otto Frisch, who had co-authored an early memorandum showing that only a small amount of uranium was needed for a weapon, led the Critical Assemblies group. James Tuck helped develop the precisely shaped explosive lenses required for the implosion-type bomb. Rudolf Peierls, Frisch’s collaborator on that early memorandum, used his expertise in blast wave physics to solve complex mathematical problems central to the weapon’s design. William Penney calculated the optimal detonation altitude for the bombs that would be dropped over Japan and worked with American physicist Luis Alvarez to predict blast damage.
James Chadwick, the British physicist who had discovered the neutron in 1932, stayed in Washington during the war but played a crucial diplomatic role, maintaining the relationship between the British government and General Groves that kept the international partnership running smoothly.
The Industrial Sites That Produced the Fuel
Los Alamos designed the bomb, but it could not function without fissile material: enriched uranium and plutonium. Producing these materials was the single most expensive part of the entire project, and it required two enormous industrial complexes built from scratch in remote locations.
Oak Ridge, Tennessee, known officially as the Clinton Engineer Works, was dedicated to uranium enrichment. The site housed multiple production plants using different technologies: gaseous diffusion, electromagnetic separation, and thermal diffusion. Construction on a massive enrichment building began in the fall of 1942. By early summer 1945, Oak Ridge employed almost 50,000 workers. The Oak Ridge facilities alone consumed over $1.1 billion of the project’s budget, more than 60% of total spending.
Hanford, Washington, known as the Hanford Engineer Works, was built to produce plutonium. The site needed enormous quantities of water for cooling its nuclear reactors, making the remote stretch along the Columbia River an ideal location. Despite a construction workforce rivaling Oak Ridge’s, Hanford’s operational staff was much smaller, around 6,000 people, because plutonium production relied more on reactor operations than hands-on processing. By 1944, both sites were well on their way to delivering the materials Los Alamos needed.
The Workforce Most People Never Hear About
The Manhattan Project’s 129,000-person peak workforce in June 1944 included far more laborers, technicians, and support staff than scientists. Construction workers built entire secret cities. Machinists fabricated precision components. Clerks, nurses, teachers, and librarians kept those cities running. The vast majority of these workers operated under strict compartmentalization, meaning they knew only their own small task and nothing about the project’s ultimate purpose.
Thousands of women filled roles that would traditionally have gone to men during a time of severe wartime labor shortages. Young women operated the control panels of calutrons at Oak Ridge’s Y-12 electromagnetic plant, manually adjusting dials to separate uranium isotopes. They welded, processed war bonds, monitored instruments, and worked as secretaries and technicians. Women also held scientific positions. At Los Alamos, the project’s leaders recruited anyone with something to contribute regardless of gender, age, or whether they held a PhD or simply had technical experience.
The workforce was not exclusively white. Aggie Lee, a University of New Mexico graduate whose parents were Pueblo Indian and white, joined the hematology laboratory at Los Alamos, collecting and analyzing blood samples from researchers exposed to radioactive materials. She was one of the few Native Americans working on the project, a reminder that the bomb’s construction drew on a wider cross-section of American society than popular accounts typically suggest.
Where the Money Went
The Manhattan Project’s spending reveals what actually mattered most to building the bomb. The grand total through December 31, 1945 was approximately $1.89 billion in wartime dollars. Oak Ridge consumed the lion’s share: over $1.18 billion, split between the K-25 gaseous diffusion plant ($512 million), the Y-12 electromagnetic plant ($478 million), and headquarters, utilities, and smaller facilities. Hanford cost $390 million. Los Alamos, the laboratory where the weapon was actually designed and assembled, cost just $74 million, roughly 4% of the total.
This breakdown makes a point that often surprises people. The hardest and most expensive part of building the atomic bomb was not the physics or the weapon design. It was the industrial challenge of producing enough enriched uranium and plutonium. The science was essential, but the manufacturing is what consumed the money, the labor, and the years.
Two Bombs, Two Designs
The Manhattan Project produced two fundamentally different weapons. The first, called “Little Boy,” was a uranium bomb that used a gun-type design, firing one piece of enriched uranium into another to trigger a chain reaction. Scientists were confident enough in this design that it was never tested before being dropped on Hiroshima on August 6, 1945.
The second, called “Fat Man,” was a plutonium bomb that used a far more complex implosion design. A sphere of plutonium was surrounded by carefully shaped explosive lenses that, when detonated simultaneously, compressed the core to a critical density. This was the design tested at the Trinity site in New Mexico on July 16, 1945. The shaped explosive lenses that made implosion work were one of the project’s most difficult technical achievements, requiring contributions from both American and British specialists. Fat Man was dropped on Nagasaki on August 9, 1945.
So who built the atomic bomb? A general who made fast decisions and a physicist who could manage geniuses. A few hundred of the world’s best scientists, including émigrés who had fled fascism in Europe. Tens of thousands of workers who operated machines they didn’t fully understand, in secret cities that didn’t appear on any map. And an industrial base that turned theoretical physics into 130,000 pounds of factory equipment, reactor coolant, and processed uranium, all in under three years.