The XP-8 was a Boeing Model 66 modified with a Packard inverted (cylinder heads down) engine. The XP-8 appearance was distinguished by the incorporation of the radiator into the lower wing root and contoured engine cowling. Only one XP-8 was built (S/N 28-359). The aircraft never entered production because of continuing problems with engine oil and water seeping to the bottom of the engine and fouling the spark plugs.