What was the name of the US project to build the atomic bomb?

The U.S. project to build the atomic bomb was called the Manhattan Project. It was a secret Allied wartime research and engineering program that developed the first nuclear weapons during World War II.

The project was formally organized in 1942 under the U.S. Army Corps of Engineers' Manhattan Engineer District, although its facilities were spread across the United States and Canada. Major sites included Oak Ridge, Tennessee, for uranium enrichment; Hanford, Washington, for plutonium production; and Los Alamos, New Mexico, where scientists designed the weapons under J. Robert Oppenheimer.

The first nuclear device, nicknamed the Gadget, was detonated at the Trinity test site in New Mexico on July 16, 1945. The project then produced the uranium bomb dropped on Hiroshima on August 6 and the plutonium bomb dropped on Nagasaki on August 9. Japan announced its surrender on August 15, 1945.

Los Alamos Project is a common mix-up because Los Alamos was the central weapons-design laboratory, not the name of the entire undertaking. The Dayton Project was a related wartime effort that produced polonium initiators, while Operation Paperclip was a postwar program for recruiting German scientists and engineers.