It was developed in 1944 as an improvement to the SU-85, built on the same chassis as the T-34-85 tank. It was designed and built at the UZTM in Yekaterinburg. The SU-100 quickly proved itself to be among the best self-propelled anti-tank guns of World War II, able to penetrate 125 mm of vertical armor from a range of 2,000 m and the sloped 85 mm front armor of the German Panther from 1,500 m . After intensive testing with different models of 100 mm gun Soviet engineers approved the D-10S gun for mass production. The hull of SU-100 had major improvements over the SU-85; the thickness of the front armour was increased from 45 to 75 mm , and the commander's workplace was made in a small sponson on the right side of the hull; combined with the commander's cupola this greatly improved the commander's effectiveness. For better ventilation two ventilator units were installed, instead of only one as in the SU-85. Mass production began in September 1944.