Staff Writer (Updated: 5/16/2016):
As soon as the M10 entered serial production, the US Ordnance Department began looking into up-gunning the M10 series and trialled a 90mm M3 high-velocity anti-tank gun mounting with the M10A1 design (made from the Sherman M4A2 hull). However, the M10 turret, as it stood, was not intended for such a gun so a new turret design was initiated. It its modified form, the vehicle came under the prototype designation of "T71 GMC" (GMC = "Gun Motor Carriage"). In June of 1944, the T71 was officially designated as the "M36 GMC" which began deliveries to the European battlefronts by the end of the year. Another variant based on the Sherman M4A3 hull and mounting the 90mm gun were converted for the tank destroyer role to become 187 examples of the "M36B1". The similar "M36B2" provided the 90mm gun turret atop the M4A2 chassis and M10 hull with power from a diesel engine. 287 of this type were converted as such. Like other armored American vehicles lacking any sort of imaginative name, the British stepped in to nickname the M36 the "Jackson" after famed American Civil War General "Stonewall Jackson" (consistent with the M5 General Stuart, M3 General Lee/Grant and M4 General Sherman). To others, it was simply known as the "Slugger". Approximately 1,772 M36 examples of all types were eventually completed - either as new-build or as conversion models. Of these, 1,298 were made up of the original base M36 models (M10A1 hull / M4A3 chassis).
The M36 formally replaced the M10 series in the US Army inventory. As in the M10 before it, the M36 was completed with an open-topped turret to save on weight. This also allowed much needed head room for the gunnery crew in the turret while supplying the tank commander with unobstructed views of the action ahead. However, this also opened the crew to the dangers of warfare as well as the elements . An optional folding armor roof kit was issued to provide some level of point protection for the turret crew.
Primary armament centered around the 90mm M3 series main gun. The American 76.2mm was always considered a step below the British gun of same caliber and even the German 75mm. The British Army even changed the 76.2mm main guns of their arriving Lend-Lease American M10s to British 76.2mm anti-tank types. As such, the 90mm caliber was a necessity for the newer American design in an effort to penetrate the front stout armor of German Panthers and Tiger Is and, upon inception of the M36 into service, the Allies finally had a weapon system capable of engaging these powerful enemy tanks. Of course the usual flanking maneuver was still in order - attacking the sides and rear of these German beasts to help achieve uncontested direct hits to the more vulnerable sides - this even before the slow-reacting turrets of the Panthers and Tiger Is could counter the threat. High velocity armor-piercing ammunition appearing later in the war only served to improve the inherent penetrative powers of the M3 gun. 47 projectiles of 90mm ammunition were stowed within the vehicle, most of these within easy reach. The large structural overhang at the rear of the turret worked to stow some of these projectiles while also doubling as a counterweight of sorts for the 90mm gun.