Maybe a stretch of the thread.
Three
Boeing 299Z ships were converted from surplus B-17G stock by Boeing. They were used by Allison, Curtiss-Wright, and P&W as engine testbeds. All three ships were, on occasion, flown in cruise with the four R-1820s shut down, so the only power was the single testbed engine.
Note that P&W's ship, NC5111N, was retired to the Bradley Air Museum (or New England air museum) and damaged in a tornado in the late 70s. This ship eventually was reconverted to B-17G, and became "Liberty Belle", flying from 2005-2011 for the Liberty Foundation. It was destroyed by fire after a forced landing two months ago.