An engine fire is not something to be trifled with in a radial piston airplane like a B-17. It can turn catastrophic VERY fast.
Oh yes, I concur. Still unclear if there was a fire, it was my first thought when the crew said "got #4" "blow it out" on the radio. My mind went to "got" a fire and "blowing" the fire bottles. But with more info I think that that may have been an incorrect speculation.
Read today that the F.E. survived, I think that should make it a pretty straightforward investigation, as no one would know better then him what those engines where doing, and fuel testing shouldn't take long.
I happen to know a bit about big radial powered airplanes.