The Boeing B-50D. The B-50D was a vastly improved version of the famous B-29, fitted with Pratt & Whitney R-4360 “corncob” engines. The flight engineer on the B-50s I flew always monitored the engines closely on a system that revealed how the spark plugs, valves, etc. were functioning. If anything looked suspicious they would suggest that the engine be feathered. The aircraft commander would decide how much further to carry on the mission, depending upon location, requirements, etc, or could elect to make a precautionary landing. It was far easier on the engines, and of course, much better than having them fail in flight, where there was always a possibility of a catastrophic failure or an engine fire. When retired from their role as a bomber by the arrival of the B-47, B-50s were converted into KB-50 tankers.