There’s a danger of confusing military aircraft and the term ‘warbird’.
I think, to be a warbird, an aircraft must have (a) been built exclusively for the military (not just a military version/variant)
and (b) either have been designed as a fighter or bomber or, at the very least, have been used in action in some role.
The Boeing / Stearman PT-17 was designed as a military trainer (basic) and was certainly successful in that role but, as far as I know, never saw action in any capacity. It’s a lovely aircraft to fly, and has many qualities (eg relatively forgiving, beautiful) and I'd love to think of it as a warbird but, can a trainer really be described as a warbird if it wasn’t in action in any role?
The T-6 (Harvard/Texan/SNJ), ‘the pilot maker’ was designed as a low cost advanced trainer with all the characteristics of a high-speed fighter, and is most famous for that role, but it also won honours in World War II. Then, in the Korean War, it was used extensively for forward airborne control, initially near the front lines and later further into enemy territory. A much stronger claim and, as far as I was concerned, my Harvard was unarguably a warbird. Well, perhaps not unarguably – some of the guys who flew the fighters occasionally smiled tolerantly at the idea of an aircraft which didn’t fight or drop bombs being a ‘warbird’.
Chipmunk a warbird? A superb trainer, but I’d be hard pushed to say I did my basic training in a warbird - even though it was an RAF Chipmunk. Still, if Chipmunks did see action in Malaya or Borneo, perhaps I need to revise my my view.
MLS-12D asks if there’s any harm in being as inclusive as possible. No harm – it doesn’t really matter - but it does make the term fairly meaningless if it’s too inclusive.
Where do you stop? Any aircraft used by the military? A military variant of an aircraft also manufactured for civil use?