The Boeing B-50 Superfortress is an American strategic bomber. A post–World War II revision of the Boeing B-29 Superfortress, it was fitted with more powerful Pratt & Whitney R-4360 radial engines, stronger structure, a taller tail fin, and other improvements. It was the last piston-engined bomber built by Boeing for the United States Air Force, and was further refined into Boeing's final such design, the prototype B-54. Although not as well known as its direct predecessor, the B-50 was in USAF service for nearly 20 years.After its primary service with Strategic Air Command (SAC) ended, B-50 airframes were modified into aerial tankers for Tactical Air Command (TAC) (KB-50) and as weather reconnaissance aircraft (WB-50) for the Air Weather Service. Both the tanker and hurricane hunter versions were retired in March 1965 due to metal fatigue and corrosion found in the wreckage of KB-50J, 48-065, which crashed on 14 October 1964. Lucky Lady II is a United States Air Force Boeing B-50 Superfortress that became the first airplane to circle the world nonstop. Its 1949 journey, assisted by in-flight refueling, lasted 94 hours and 1 minute. The plane later suffered an accident, and today only the fuselage is preserved.