These planes were built to survive a nuclear air blast. They are really tough, and the airframes were just tested for durability, and cleared to fly until 2097. That is about 150 year lifespan, and might be extended again.
It shows you how building something for worst conditions is often better than building something to perform optimally. The boeing 777, and 737max are not going to last 30 years, and with the B-52, a great grandchild could be the pilot of the same plane his grandfather flew. Incredible.
Listen to Bill Whittle discuss here: