With the end of fighting in Korea, President Dwight D. Eisenhower, who had taken office in January 1953, called for a "new look" at national defense. The result: a greater reliance on nuclear weapons and air power to deter war. His administration chose to invest in the Air Force, especially Strategic Air Command. The nuclear arms race shifted into high gear. The Air Force retired nearly all of its propeller-driven B-29/B-50s and they were replaced by new Boeing B-47 Stratojet aircraft. By 1955 the Boeing B-52 Stratofortress would be entering the inventory in substantial numbers, as prop B-36s were phased out of heavy bombardment units rapidly.