Staff Writer (Updated: 7/1/2014):
The lackluster draw of the SU-122 led Soviet authorities to continue their search for a dedicated tank-killing system. As a starting point, and to facilitate grand scale production, the hull of the SU-122 (itself a copy of the T-34 hull) was utilized and an existing 85mm anti-aircraft cannon was fitted. Utilization of these components ensured that the new weapon system could be produced quickly and in the required numbers while keeping expenses under control. A prototype was made available in August of 1943, evaluated and accepted into production as the SU-85 to which 750 examples were delivered by the end of the year.
By this time, the original 76mm main gun of base T-34 tank models were proving rather ineffective against the new generation of German tanks. The 85mm gun proved the next logical step for the series and the new T-34/85 was born. In effect, the arrival of the T-34/85 rendered the dedicated SU-85 somewhat redundant which led to production of the latter wrapping up in September of 1944, completing the delivery of some 2,000 examples.
As the SU-85 was on its way out, its replacement became the SU-100 mounting the more powerful 100mm main gun. The SU-100 entered service the same month that the SU-85 was discontinued.