I'm going to be real vague and say this is part of the engine. If you're vague enough, you're always right. The B-17 first flew as the YB-17 in 1935 and this looks like something that was designed in the thirties-- a combination robot and sea creature.
	From reader Tom Griffith-- Your photo of the GE Type-B Turbocharger is really good.  Being an aircraft engine and propeller nut, I've taken photos of engines and props of every warbird that I've ever met.  The turbos on the B-17s, B-24s, P-38s (when they've been left on by restorer/rebuilders), etc make for interesting photos.  I especially liked your description in that photo as being part robot and sea creature.  Maybe you asked the crew, but in case you didn't, your photo is of the exhaust turbine part of the unit.  I can see that the waste-gate is open, so the exhaust gases will, well, exhaust themselves from the engine without going through the turbine blades or buckets themselves.  This winds up NOT providing boost to the engine at altitude, etc, etc.  I've found out over the years that very few of the turbos on warbirds actually ever are "allowed" to do their job and produce boost, because the planes are neither flown at high altitudes, nor are they every carrying anything (oh, I don't know, bombs!) to put them near their maximum weights, etc, etc.  I've seen waste-gates welded open so that the turbo isn't accidentally told to do its job.  Didja know that the turbocharger on the P-47 is on the underside of the rear fuselage, just in front of the rear landing gear?  The shape of the P-47 starting at the nose and going all the way to the back is generally dictated by the air and exhaust plumbing that travels between the R-2800 engine and the GE turbocharger.  My favorite P-47 "Tar Heel Hal" at LSFM, has had its turbo removed (and I guess a lot of weight put there to replace it - it's a larger one than you saw in the B-17) and all of the exhaust gas exits from pipes behind the nose. An interesting story about turbo blades/buckets:  a friend of mine, who died this year, was a B-24 and B-29 pilot in WWII.  He said that on the B-24, since it had a high wing and this put the turbos even with the fuselage, if a blade broke off when the waste-gates were closed and the turbos were spinning at > 100,000 RPMs, the blades naturally were thrown out from the turbo and you could hear them hitting the paper-thin fuselage of the Liberator!  All of this when you're trying to concentrate on dropping bombs without being knocked out of the sky!  Sure, the individual blades are tiny, but the sound HAD to be unnerving if nothing else!