The M26 Pershing was the culmination of the T20 series of test vehicles. The T26 series was reclassed from medium to heavy tanks on June 29, 1944, but renamed as medium tanks in May 1946. Although not standardized until March 1945, Pershings had been sent to the European Theater of Operations as T26E3s with the Zebra Mission in January 1945 (along with equipment including pilot models of the 155mm GMC M40 and 8" HMC M43). M26 was armed with the same 90mm gun as the M36 tank destroyer, and the GAF engine in the M26 was essentially the GAA of the medium tank M4A3 redesigned to present a lower height, even though M26 weighed almost 26,000lbs (12,000kg) more than M4A3. This produced an underpowered and relatively unreliable tank. The drive sprocket of the M26 was mounted below the level of the track's upper run, and the engine exhaust escaped through an opening in the hull rear plate. A bulge in the hull glacis between the drivers housed an exhaust blower.

In 1948, the first improved M26A1s were accepted. These tanks were armed with the 90mm gun M3A1 in the combination gun mount M67A1. The M3A1 was fitted with a bore evacuator and a lighter, single-baffle muzzle brake. The M67A1 mount had a lighter equilibrator spring since the M3A1's lighter muzzle brake imparted less torque upon firing. M26A1 also had a modified gun travel lock, and some vehicles were fitted with a gun elevation stabilizer.