The oldest B-52 on display is at the Pima Air & Space Museum in Tucson. It is the 3rd ‘A’ model built and was the mothership for the X-15 program. It was used in 90 X-15 launches.
https://pimaair.org/museum-aircraft/boeing-b-52a/
B-52 (52-0013) is at the Atomic Museum in Albuquerque, it was used extensively for nuclear testing in the Pacific, being the first 52 to drop a thermonuclear weapon. It was cosmetically restored recently very accurately.