The B-50 was a later development of the Boeing B-29 Superfortress. The original designation was "B-29D", but the designation was switched because the Air Force (rightly) believed congress in 1945 would not fund further development of a WWII-era design, no matter how advanced.
While superficially identical to the B-29, the B-50 was actually quite different. In its primary role, it mostly served as a stop-gap between the obsolete B-29 and the jet-powered B-47. It was also employed in many other roles, such as reconnaissance and air-to-air refueling. The B-50 was retired from its main role as atomic bomber in 1955, and was removed completely from inventory ten years later. No flying examples exist today, although several can be found in various air museums.