Well…looks like the B-52 can potentially fly until the year 2097 if need be.
Captain’s Log, Stardate 43125.8.
In preparation for the decommissioning ceremony for the USS Enterprise NCC-1701D, we have the exceptional honor of being escorted by one of the newly re-engined Federation ships…the B-52X.
First flown April 15th 1952, the B-52 has been in continuous service, undergoing multiple upgrades during its lifetime. The B-52X is now expected to serve up to stardate 53000.0 and beyond if no suitable replacement is available at that time.
Johnny, you totally win the thread by several megatons worth.
I stole it. 
Hear hear.
I wish I myself had the answer to the question. So many other planes have been decommissioned. What singles out the B-52H as being potentially airworthy until 135 years after the last ones were built?!
I explained why several posts up. Looks like Dale Brown’s Old Dog could be a reality… 
They are heavily overbuilt compared to AC designed today, and aircraft maintenance essentially tears the plane down and rebuilds it during depot level periodic inspections.
I wish there was a better way to give kudos to a great post so:
There are still at least 172 DC-3s licensed to fly, and given the number of intact models I’ve seen in museums, there are probably a lot more that could be made flyable without much effort. The last DC3/C-47 came off the assembly line in 1945.
The DC3, as I remember, did not have a pressurized cabin (note the square windows in pictures). So it wasn’t subject to the same cyclic stress.
Also, IMHO, being new in the shift from timber to metal structure, it was over-engineered.
The B-52 is also overengineered. That’s why they’re both still flying.
As to pressurization, the only part of a B-52H that’s pressurised is the cockpit. So from the windshields after for about 20 feet. The whole rest of the airplane is not.
I too am surprised they’ve held up as well as they have. The vertical tails started breaking off the D models back in the mid 1960s, so first the tails were all replaced then later the D-models were all scrapped.
Wasn’t there an aft-gunner position? Was that unpressurized? (I really do not know)
When I say, “I really do not know” in this context I mean I do not know if the tail gunner position was pressurized. I do know there was a tail-gunner in the original design.
The early models had a pressurized tail gunner compartment. All those jets have long since been chopped up.
The surviving B-52H model has a tail gun. But the gunner sits facing aft right behind the aircraft commander in the cockpit at the front of the jet.
They are gone.
" General George Lee Butler announced that the gunner position on B-52 crews would be eliminated, and the gun turrets permanently deactivated, commencing on 1 October 1991." Related to a US anti-radiation missile HARM locking onto the radar from the tail gun. Boeing B-52 Stratofortress - Wikipedia
One of the reasons is because modern military aircraft are designed by government committees, and each person wants the plane to have a “special” feature (which will later show up on the person’s resume). So then you end up with a plane that can do everything & anything, resulting in massive cost overruns and delayed schedules (e.g. F-35).
And that’s why Skunk Works was so successful… Lockheed kept the government at bay. The government gave Lockheed general specs in the beginning, and then Lockheed came up with the final product. (That’s a gross oversimplification, but you get the idea.)
Why the difference in square and round windows in a pressurized fusalage?
The square has corners that concentrate stress, such as the recurring stretching and relaxing of the skin during climbing to altitude and descending. That concentration accumulates stress which becomes localized accelerated metal fatigue, and before you know it the window blows out (taking large chunks of hull with it) and wrecks the plane.
Was there any way to get from the tail gunner’s compartment to the main cabin? If not, that would have been a seriously lonely job.
IIRC the answers are “No” and “Yes”.
The earlier B-29 & B-36 had crawl tunnels between the various aft gunner positions and the cockpit area. But not the B-52.