The Boeing B-29 Superfortress is a four-engine propeller-driven heavy bomber designed by Boeing and flown primarily by the United States during World War II and the Korean War. The Superfortress was designed for high-altitude strategic bombing, but also excelled in low-altitude night incendiary bombing. One of the largest aircraft of World War II, the B-29 had state-of-the-art technology, including a pressurized cabin, dual-wheeled, tricycle landing gear, and an analog computer-controlled fire-control system that allowed one gunner and a fire-control officer to direct four remote machine gun turrets. The $3 billion cost of design and production made the B-29 program the most expensive of the war. The B-29's advanced design allowed it to remain in service in various roles throughout the 1950s. The type was retired in the early 1960s, after 3,970 had been built. The B-29s also dropped the atomic bombs on Hiroshima and Nagasaki. The Enola Gay is a Boeing B-29 Superfortress bomber, named after Enola Gay Tibbets, the mother of the pilot, Colonel Paul Tibbets. On 6 August 1945, during the final stages of World War II, it became the first aircraft to drop an atomic bomb. The bomb, code-named "Little Boy", was targeted at the city of Hiroshima, Japan, and caused the near-complete destruction of the city. Since 2003, the entire restored Enola Gay has been on display at NASM's Steven F. Udvar-Hazy Center. The last survivor of its crew, Theodore Van Kirk, died on 28 July 2014 at the age of 93. Other B-29s have been restored. FIFI is a surviving Boeing B-29 Superfortress, and one of two that are currently flying, the other being Doc. It is owned by the Commemorative Air Force.