The U.S. Air Force ordered 102 B-52H bombers during the Cold War, with the first planes delivered in May 1961. The old eight-engined warhorse has been in continuous service ever since, flying combat missions over the former Yugoslavia, Iraq, and Afghanistan. The Air Force plans to keep flying the remaining 76 B-52Hs through 2050, with some almost certainly flying until 2061. The B-52H will be the longest serving warplane in history, serving longer than the newer B-1B and B-2A bombers.
https://www.popularmechanics.com/military/aviation/a29194843/b-52-upgrades/
https://www.popularmechanics.com/military/aviation/a29194843/b-52-upgrades/