 Robert Oppenheimer held conferences in Chicago in June 1942, prior to the Army taking over wartime atomic research, and in Berkeley, California, in July, at which various engineers and physicists discussed nuclear bomb design issues.They chose a gun-type design 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 proposal attracted little interest.