The B-52 Stratofortress

The B-52 Stratofortress began service in the United States Air Force in 1955 as a long range heavy bomber during the cold war era. Despite being a sub-sonic aircraft, the super-sonic B-1 Lancer and B-2 Spirit have failed to replace the B-52 as a long range heavy bomber with the B-52 remaining in service. The cancelled B-70 Valkerie project, despite reaching speeds of mach 3 and an operating altitude of 70,000 feet failed to replace the B-52 as the front line strategic bomber. 

The B-52 remains in service for a number of reasons, the aircraft has the ability to fly long range missions, loiter in an area just outside of the battle field before dropping precise weapons and is a robust aircraft that is reasonably cost effective to operate. The B-52 is expected to remain in service until 2045, an extremely long service life for not only a military aircraft but for any aircraft.  

The B-52 can carry an array of weapons from nuclear to conventional; during the cold war the B-52 loitered at high altitude close to the borders of the former Soviet Union providing rapid first strike or retaliation capabilities as part of a three prong nuclear deterrence strategy. Amazingly, 742 B-52 aircraft have been built with a production run from 1954 to 1963. In comparison, 104 B-1 Lancer 21 and 21 B-2 Spirit stealth bombers were built ensuring the B-52 remains the most prolific aircraft in the heavy strategic bomber fleet.