Regarding surviving early airframes I’ve seen “Balls 3” at Pima Air Museum in Tucson and “Balls 5” at Wings Air Museum in Denver. I think those two aircraft along with “Balls 8” did all the X-15 launches, although I can’t remember for sure if 8 was ever configured to launch the X-15 or if #3 and #5 did it all between them. I know “Balls 3” still has the X-15 launch pylon mounted on the starboard wing inboard of engines 5 & 6, can’t remember if “Balls 5” still does though. So maybe “Balls 3” is the oldest surviving B-52 airframe. Boeing scrapped both the XB and YB prototypes didn’t they?