Lycoming built two XR 7755-3 prototypes. Lycoming and the U.S. Army successfully tested them, but neither engine ever flew in an airframe.
Lycoming
XR-7755-3
The XR-7755-3 is the largest, most powerful reciprocating aircraft engine in the world. This 5,000 hp engine was intended to power the Convair B-36. With the cylinders four deep, air cooling was not an option, but providing liquid cooling to all 36 cylinders proved a plumbing challenge.