What was the last fighter plane capable of open cockpit flying?
In the US arsenal? A-1 Skyraider, I would guess.
The A-1 was probably the last piston-powered airplane in active US service with a traditional WWII-style canopy. BUT … the A-1 was not a fighter. It was a lumbering bomb-truck.
To the OP, it depends a bit whether you mean the last one introduced to service or the last one removed from service. Some airplanes have careers lasting a few years, others a few decades.
Of all the various US P- and F- designated airplanes, all the piston planes could be flown with the canopy at least partly open. Of the jets, I seem to remember pictures of the the F-86 with the canopy “cracked” (open a few inches) flying in the traffic pattern on a hot day.
Almost all subsequent jets had canopies which pivoted upwards, rather than sliding rearwards. Planned canopy-open flight in them is not a good idea; the wind will promptly rip it off the plane.
So I’m going to bet on the F-86. The F-86 first entered production in 1948 and the last one was withdrawn from service in 1965. http://www.marchfield.org/f86l.htm
But that’s an informed opinion, NOT a provably accurate answer to your question. Now on to the digressions …
Almost all models of jet have been flown at least once or twice without a canopy after a failure of some kind. I’ve seen pictures of F-4s, F-100s, USN F-8s, etc., all missing most or all of the canopy.
I flew the F-16, where the canopy and the windshield were all one piece. It made for great visibility, but if it ever came off you’d essentially be sitting on a multi-hundred MPH motorcycle. The HUD (gunsight) was especially strengthened to act as a last-ditch windscreen to protect you long enough to eject. Or so they told us.
I heard that during development they tried one test flight without the canopy installed just to see how much, if at all, they could fly it. At the slowest speeds the plane could take off and land (125mph-ish), it was tolerable. As the test pilot accelerated gingerly towards 150, the wind tried to pull his arms out of the cockpit, which would have been fatal. So he promptly slowed down & landed.
Yikes. I’m not vouching for the truth of that story, only repeating what I was told. Those guys have brass balls, but that sounds a little too risky even for them.
The spad (the Skyrader, not the French WWI biplane) had a sliding canopy but wasn’t meant to be open for most flying. This site says the Boeing P-26 Peashooter was the last US open cockpit fighter. It was the first all metal monoplane adopted by the US Army so it bridges eras. Another contender is the British Gloster Gladiator biplane which was introduced in '35.
On at least one model of the Bell P-59 Airacomet, (America’s first jet fighter), there was an open cockpit seat (with a windscreen), which was placed ahead of what would be the pilot’s cockpit (which had a canopy.)