Looking at pics I guess it didn't take much for the aircraft skin start buring (alumunium?) and then spread quickly?
Fuel leak in one of the starboard engines spread to the rest of the fuel system, causing an explosion amidships. Apparently the leak had been a recent nagging problem that was never quite nailed down, but we'll see what the FAA rules on this. There could be a fine for the owner for flying a non-airworthy (again, due to the leak) plane.
Sheet aluminum doesn't burn under normal conditions. The damage to the wings you see were due to the explosion. Close up photos will show the parts of the plane that were fabric, which did burn. The paint on the aluminum parts appears to be charred, but did not burn.
Sheet aluminum may not burn, but both it and the aluminum airframe will melt, and looking at the
photos in this set, that's what seems to have happened in this fire. I'm not 100% certain, but I have my doubts that any fabric skin was used on the B-17, with the possible exception of that on the flaps and the vertical and horizontal stabilizers.
Also, I found a 10-minute video clip from a flight the plane made back in 2008, from Coatesville, PA to Albany: