 In 1942 , prior to the Army taking over wartime atomic research , Robert Oppenheimer held conferences in Chicago in June and Berkeley , California , in July , at which various engineers and physicists discussed nuclear bomb design issues . A gun @-@ type design was chosen , in which two sub @-@ critical masses would be brought together by firing a " bullet " into a " target " . Richard C. Tolman suggested an implosion @-@ type nuclear weapon , but the idea attracted scant consideration . 
