This is the B-17 auxiliary power unit. Basically, this is a small engine that sits in the back of the plane, and provides power via generator to the aircraft systems. This particular APU is probably a two-stroke, since you can see what looks like an air filter feeds the crankcase. If you look closely, you can see that it was manufactured by Homelite, the company that exists today and makes a lot of weed whips.
The Boeing B-29 had a four-stroke APU that was the fore-runner to their gasoline lawnmower engines. I was told that this was built by Briggs & Stratton.
