As a result of a desire by the Armored Forces Board to install a 90 mm gun into the M4A3 Medium Tank the Ordnance Department approved the building of pilot models in May 1942.
A wooden mock-up was made by Fisher and was approved by the Ordnance Department in September 1942.
Three prototypes were ordered, and each had a different turret and gun for testing.
The turret on the T20s was cast. The loader had a rectangular double hatch.
In July 1943 the Ordnance Department proposed that the T20E3 would be standardized as the M27B1 Medium but this was denied.
The T22 prototype was authorized for construction in late 1942. They were similar to the T20E1.
The T22E1 had an automatic loader installed which moved the commander to the left side. The automatic loader didn't function well.
The biggest problem the T20s and T22s had was that the transmission wasn't reliable.