The North American B-45 Tornado was the first jet bomber to enter service with the United States Air Force. It was designed and built by North American Aviation in the late 1940s, as a response to the need for a fast, long-range bomber that could deliver nuclear weapons. The B-45 had four jet engines mounted in pairs under the wings, and a swept-back tail. It could carry up to 22,000 pounds of bombs, including the Mark 7 nuclear bomb. The B-45 was also capable of aerial refueling, which extended its range and endurance. The B-45 first flew in March 1947, and entered service in 1948. It was initially assigned to Strategic Air Command (SAC), but later transferred to Tactical Air Command (TAC) in 1950. The B-45 saw combat during the Korean War, where it performed strategic bombing and reconnaissance missions. It also participated in several covert operations over the Soviet Union and China, such as Operation Ju-Jitsu and Operation Home Run. The B-45 was gradually replaced by the more advanced Boeing B-47 Stratojet in the mid-1950s, and retired from service in 1959. A total of 143 B-45s were built, of which only one survives today at the National Museum of the United States Air Force.