High Quality Content by WIKIPEDIA articles! The Medium Tank T20 was part of a series of medium tanks designed by the United States during the Second World War, to be the successor to the Medium Tank M4 "Sherman", and culminated in the M27. Almost immediately after the M4 Sherman was standardized, the US Ordnance Department started work on a successor. The project was started on the 25 May 1942, it was originally designated M4X. Starting with the T20, the Ordnance Department initially developed three series of improved medium tank prototypes.