We still use the B52 as well as the U2 (remember Gary Powers?) since 1955. There are three more with the oldest from 1949. https://www.veteranaid.org/blog/5-oldest-us-aircraft-still-in-operation#:~:text=5%20Oldest%20U.S.%20Aircraft%20Still%20in%20Operation%201,KC-135%20...%204%20U-2%20...%205%20U-6A%20