The atomic bomb grew out of a single laboratory discovery in Berlin in late 1938, scaled up through an unprecedented wartime industrial effort, and culminated in the first nuclear detonation just six and a half years later. The project that built it, code-named the Manhattan Project, employed over 125,000 people across three secret cities, cost nearly $2 billion in 1945 dollars (roughly $21.5 billion adjusted for inflation), and produced two fundamentally different weapon designs. Here’s how it happened, from the physics breakthrough to the final test.
The Discovery That Started Everything
In December 1938, German radiochemists Otto Hahn and Fritz Strassmann were bombarding uranium with neutrons in their Berlin laboratory when they noticed something no one expected. Instead of changing slightly under neutron bombardment the way most elements did, uranium nuclei broke into two roughly equal pieces, releasing enormous energy. The physicist Otto Frisch, borrowing a term from cell biology, named the process “fission.”
Two details made fission more than a scientific curiosity. First, splitting a uranium nucleus released far more energy than any chemical reaction ever observed. Second, the process also emitted additional neutrons, which meant those neutrons could strike other uranium nuclei, triggering a self-sustaining chain reaction. If a chain reaction could be controlled, it could generate power. If it could be left uncontrolled, it could produce an explosion of staggering force. Italian physicist Enrico Fermi had actually produced fission back in 1934 without recognizing what he’d done. Now the implications were impossible to miss, especially with war looming in Europe.
The Letter That Reached Roosevelt
Hungarian-born physicists Leo Szilard, Edward Teller, and Eugene Wigner understood immediately that Nazi Germany, where fission had been discovered, might try to build a weapon based on the principle. They regarded it as their responsibility to alert the American government. In the summer of 1939, they persuaded Albert Einstein to sign a letter to President Franklin Roosevelt warning that recent fission research made the construction of “extremely powerful bombs” conceivable, and that Germany appeared to be actively pursuing this work.
Roosevelt was initially noncommittal and worried about finding the necessary funds. But at a follow-up breakfast meeting, he became convinced. On October 19, 1939, he wrote back to Einstein, informing him that he had established a committee of civilian and military representatives to study uranium. That decision, driven by the fear that Hitler might achieve sole possession of a nuclear weapon, set in motion the chain of events that would become the Manhattan Project.
Three Secret Cities, Three Different Jobs
By 1942, the effort had grown far beyond a committee. Army General Leslie Groves took command of what was now a sprawling industrial and scientific enterprise, and he approved three primary locations, each with a distinct role.
Oak Ridge, Tennessee was built to enrich uranium. Natural uranium is overwhelmingly composed of uranium-238, which doesn’t sustain a chain reaction. The rare isotope uranium-235, making up less than 1% of natural uranium, does. Separating the two was the central challenge. Oak Ridge housed multiple enrichment plants, including the massive K-25 gaseous diffusion facility. The process worked on a simple principle: when uranium was converted to a gas and pushed through porous barriers, the slightly lighter uranium-235 molecules passed through more readily than the heavier uranium-238. But the difference was tiny, so the gas had to pass through hundreds of cascades in sequence, each stage concentrating the uranium-235 a little more. The K-25 building alone measured half a mile by 1,000 feet, larger than the Pentagon, yet it produced its final product in quantities measured in grams. The demand for construction workers swelled Oak Ridge’s population from a planned 13,000 to over 75,000 by war’s end.
Hanford, Washington produced plutonium, the second path to a bomb. Plutonium doesn’t exist in nature in useful quantities. To create it, engineers built nuclear reactors where natural uranium fuel sustained a controlled chain reaction. Inside these reactors, uranium-238 captured neutrons and gradually transformed into plutonium-239, a new element that could also sustain an explosive chain reaction. Construction of Hanford’s first reactor, the B Reactor, began in October 1943. The uranium fuel had to be meticulously prepared: metal ingots were heated to over 900 degrees Celsius, extruded into rods, machined into precise slugs, sealed in aluminum cans, and welded shut, all to ensure proper heat transfer and minimize unwanted neutron absorption inside the reactor.
Los Alamos, New Mexico was where the weapons themselves were designed. Perched on an isolated mesa, the laboratory gathered some of the world’s most accomplished physicists under the direction of J. Robert Oppenheimer. Their job was to take the enriched uranium from Oak Ridge and the plutonium from Hanford and figure out how to turn those materials into functioning bombs.
Two Bomb Designs for Two Different Materials
The scientists at Los Alamos developed two distinct weapon designs because the two fissile materials behaved differently.
The uranium bomb, called Little Boy, used a gun-type design. It was the simpler of the two. An explosive propellant fired one piece of uranium-235 down a barrel into another piece. When the two subcritical masses slammed together, they formed a single mass large enough to sustain an explosive chain reaction. As retired Los Alamos weapons scientist Glen McDuff later explained, Little Boy “could be tested without an explosive test and was guaranteed to work.” That guarantee mattered, because building one Little Boy consumed virtually all the purified uranium-235 that existed at the time. There was no material left for a test. The finished weapon weighed 9,700 pounds, was 10 feet long, and just over 2 feet in diameter.
The plutonium bomb, called Fat Man, required a far more complex approach called implosion. Plutonium’s nuclear properties meant a gun-type assembly wouldn’t work; the material would begin reacting and blow itself apart before a full chain reaction could develop. Instead, the design surrounded a solid plutonium core with a spherical shell of carefully shaped explosives. When detonated simultaneously from all directions, these explosives would compress the core inward to a much higher density, triggering the chain reaction.
The engineering challenge was extraordinary. The explosive shell consisted of 32 precision-cast lenses (20 hexagonal, 12 pentagonal) fitted together in the same pattern as a soccer ball, forming a sphere 140 centimeters wide and weighing at least 2,500 kilograms. Each lens paired a fast explosive with a slow explosive, shaping an expanding shock wave into a perfectly converging one aimed inward. The pieces had to fit together with less than 1 millimeter of error. Any irregularity in the shock wave could ruin the symmetry of the compression and prevent the bomb from working. Because this design was so complex, it had to be tested before anyone could trust it in combat.
The Trinity Test
That test took place at 5:30 a.m. on July 16, 1945, at a remote stretch of the Alamogordo Bombing Range in New Mexico, a site known as the Jornada del Muerto, or “Journey of Death,” about 210 miles south of Los Alamos. It was the world’s first nuclear detonation.
Most scientists at Los Alamos had predicted a yield around 5,000 tons of TNT equivalent. About 40 seconds after the explosion, Enrico Fermi stood up, dropped pre-prepared slips of paper into the blast wind, and estimated from their deflection that the bomb had released energy equal to 10,000 tons of TNT. The final calculated yield was 21,000 tons, or 21 kilotons. It was more than twice Fermi’s estimate and four times what most of the physicists had expected.
Secrecy at an Industrial Scale
One of the most remarkable aspects of the Manhattan Project was how it stayed secret despite its enormous size. General Groves considered compartmentalization “the very heart of security.” Each worker was told only what they needed to know to perform their specific task. At Oak Ridge, thousands of employees operated enrichment equipment daily without any real knowledge of what they were producing. The same principle applied across all three sites. Groves also ran his own autonomous intelligence and counterintelligence operations, making the Manhattan Project function as a virtual government agency of its own.
The result was that a project employing tens of thousands of people across multiple states, consuming vast amounts of electricity and raw materials, remained largely unknown to the American public and to most members of Congress until the bomb was used. Little Boy was dropped on Hiroshima on August 6, 1945. Fat Man followed over Nagasaki three days later. From Einstein’s letter to the bombings, the entire effort had taken less than six years.