Much like other armies at the time, the U.S. Army envisioned two main roles for tanks: infantry support and breakthrough exploitation. From 1942 until the end of World War II, both roles were covered in the main by the M4 Sherman, which was better suited for the latter "cavalry" role. The infantry would have preferred a better protected and better armed vehicle, even at a price of less mobility. In late 1942, U.S. Army Ordnance started to work on an "infantry-oriented" design which was supposed to be more versatile than the British infantry tanks. During the next two years, various prototypes were built under the designations T20, T22, T23, T25, and T26. Starting with the T20, the Ordnance Department initially developed three series of improved medium tank prototypes, the T20, T22, and T23. The main differences between the T20, T22, and T23 lay in the choice of transmission. . The T20 used a torque converter fluid drive, the T22 a 5-speed mechanical drive similar to the M4 drive, and the T23 used an electric drive. Additionally, the T20 had an early version of the HVSS suspension later employed on the M4 Sherman, whereas the T20E3 had torsion-bar suspension. All moved the transmission to the rear of the vehicle, eliminating the need for a drive-shaft running the length of the vehicle. The drive-shaft used in the M3 & M4 vehicles forced the turret to be mounted higher, thus increasing the vehicle height.
T-20E3, prototype vehicle
However, the initial success of the M4 led the Army Ground Forces command to believe that there was no urgent need for a new tank. Even with the appearance of the heavy Tiger and medium Panther tanks, the AGF did not alter it's position, believing both tanks would be fielded in relatively small numbers. AGF was correct about the Tiger, a specialized heavy tank that was never encountered in large numbers. The Panther, first encountered in small numbers at Anzio, however, was built in very large numbers and formed half the German tank strength in Normandy. Also, according to the Army doctrine of the time, tanks were not supposed to engage other tanks; this was the remit of tank destroyers, more mobile armored vehicles with powerful guns, such as the M10 Wolverine. As a result, the development of the new tank was slow. When the Allies invaded western Europe during Operation Overlord in June 1944, the M4 still formed the bulk of American tank units. It quickly became clear that the tank destroyer doctrine failed in the field and that the up-gunned Sherman was unable to engage the Panther on equal terms. Efforts were made to speed up development, but the tank, by now called the T26 and dubbed Pershing, reached the battlefield only in February 1945 and saw very little action in WWII.