|(during World War II) the code name for the secret US project set up in 1942 to develop an atomic bomb|
The code name for the effort to develop atomic bombs for the United States during World War II. The first controlled nuclear reaction took place in Chicago in 1942, and by 1945, bombs had been manufactured that used this chain reaction to produce great explosive force. The project was carried out in enormous secrecy. After a test explosion in July 1945, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki.