I have a copy of the book American Secret Projects: US Airlifters 1941-1961 and it lists the following features of the M-231:
- four Allison T56 turboprops
- fuselage stretched by 5 feet
- bigger cockpit to add room for a flight engineer and optional navigator
- larger wing, rudder, and horizontal stabilizer, each with 50 percent more area
- J44 booster engines aft of the engine nacelles