Mmmh it's a gross comparison. First of all that was a B-29, which was tentatively salvaged in a borderline criminal way (and a man died for the appalling salvaging conditions); second thing, the fire spread in the tail section from a leaking fuel tank of an improvised APU installed by the team in an improbable place, nothing to do with the plane itself.

Again in this case my wild guess again is that something outside the reach of the CO2 extinguisher in the engine bay 2 failed: electric pump, supercharger or some other ancillary.

Bear in mind that the fire was apparently only spotted by a P-40 pilot flying formation with the B-17, so they had no sign on board, and it wasn't an explosive fire, so I would rule out the vapour build up in the wing box due to a leaking fuel line (another problem with B-17s).

I'm sure they'll come out with a report at some point, the FAA is always quite zealous when it comes to warbirds and safety.