Einstein's 1939 letter helped initiate the U.S. effort to build an atomic bomb, but work proceeded slowly at first. Two other findings in 1940 and 1941 demonstrated conclusively that the bomb was feasible and made building the bomb a top priority for the United States: the determination of the "critical mass" of uranium needed and the confirmation that plutonium could undergo fission and be used in a bomb. In December 1941, the government launched the Manhattan Project, the scientific and military undertaking to develop the bomb.