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In 1941, the US Army asked Caterpillar, the tractor and bulldozer company, to do something that had never been done. It wanted the Wright R-1820 Cyclone, the radial engine that powered the Boeing B-17 Flying Fortress, rebuilt as a slow-turning, high-torque diesel for the M4 Sherman tank. Caterpillar’s engineers delivered. The result was the D200A, later called Ordnance Engine RD-1820. It was a nine-cylinder, 1,823-cubic-inch multifuel radial diesel that produced 450 horsepower and up to 1,470 pound-feet of torque. It also outpulled the gasoline radial in the standard Sherman. The Army ordered 775 diesel Shermans, designated the M4A6, and the first rolled out of the Detroit Tank Arsenal in October 1943. Four months later the program was cancelled after only 75 tanks. Ordnance had decided it already had enough engine types: the Ford GAA V8, the General Motors twin-diesel M4A2 already serving the Marines, and the Chrysler A57 multibank. In Fort Knox testing that took place after the cancellation, the M4A6 recorded better fuel efficiency and range than any other standard Sherman. In this documentary we trace the full story: why American tanks ran on aircraft engines in 1941 why the Marine Corps and Navy wanted diesel how Caterpillar turned a lightweight bomber engine into a tank diesel why the Army threw it away how the Decatur plant built for 1,000 engines a month made only about 120, then switched to building the D7 bulldozers that went everywhere the Seabees went Along the way we cover the Guiberson diesel Stuart, the Ford GAA and Chrysler multibank Shermans, and the M4E1 test tanks. We also cover the final irony: Caterpillar machines served in every theater of the war, but its tank engine never left the United States, and no M4A6 or RD-1820 is known to survive today. If you enjoy in-depth World War Two engineering history, military technology, tank development, and the stories behind forgotten weapons programs, subscribe for more. Which wartime engine conversion should we cover next? Let us know in the comments, along with where you are watching from. #WW2History #ShermanTank #Caterpillar #B17 #MilitaryHistory #TankHistory #WrightCyclone #M4A6 #WWII #DieselEngine #EngineeringHistory #FlyingFortress #Seabees #MilitaryTechnology #HistoryDocumentary Sources Hunnicutt, R.P. Sherman: A History of the American Medium Tank. Presidio Press / Echo Point Books. Atkinson, Brent. “Medium Tank M4A6 Sherman.” Tank Encyclopedia. https://www.tanks-encyclopedia.com/ww... Rickard, J. “Medium Tank M4E1” and “Medium Tank M4A6.” History of War. https://www.historyofwar.org/articles... Allen, Jim. “1942 Caterpillar RD-1820 Radial Diesel.” Diesel World. https://www.dieselworldmag.com/diesel... Jackson, David D. “Caterpillar Tractor Company in World War Two.” The American Automobile Industry in World War Two. https://usautoindustryworldwartwo.com... Smithsonian National Air and Space Museum. “Wright Cyclone R-1820-97 (Studebaker), Radial 9 Engine.” https://si.edu/object/wright-cyclone-... Disclaimer: Under Section 107 of the Copyright Act 1976, allowance is made for ‘fair use’ for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. I do not own some or all of the video materials used in this video. In the case of copyright issues, please contact me at historymediachannel1@gmail.com for credit or removal. The Hunnicutt book is listed because most of the M4A6 details I used trace back to it as the standard reference. I did not consult it directly, so you can drop it and keep five sources if you prefer to list only what was read firsthand.