According to GOKO decree #3187ss issued on April 15th, 1943, and #3289ss, issued on May 5th, 1943, and orders of the People's Commissar of Tank Production and People's Commissar of Armament #233ss/260ss, issued on May 8th, 1943, Uralmash factory, factory #9, TsAKB, Kirov factory, and factory #8 were instructed to design and produce an 85 mm SPG.
TsAKB (comrade Grabin) was instructed to design the gun, factory #9 was instructed to build it, and Uralmash was instructed to build the hull and suspension.
During this project, the following guns were proposed for installation on the T-34 chassis:
- TsAKB: 85 mm S-18 gun.
- Uralmash: 85 mm S-18-1 gun, the TsAKB design with minor changes (welded mount).
- Factory #9, personal initiative: 85 mm D-5S-85 gun.
- Factory #9, personal initiative: 122 mm D-5S-122 howitzer, which has many parts in common with the D-5S-85.
In the end of July of 1943, the aforementioned SPGs were produced by factory #9 and Uralmash.
The D-5S-85 gun has the ballistics, barrel, and chamber of the model 1939 85 mm AA gun. The barrel is a monobloc type, the breech is vertically sliding, semi-automatic. The firing mechanism is of the striker type.
The recoil mechanism is positioned above the barrel. The recoil brake and return mechanism are independent mechanisms, positioned in one plane above the barrel. The brake is hydraulic, oil-filled, the return mechanism is hydro-pneumatic.
The mount is cast, clip type. The mount houses a rail on which the gun slides after recoiling.
The gun is balanced without additional weight due to increased size of the barrel and breech.
The elevation mechanism is of the sectored type, the turning mechanism is of the screw type.
The oscillating part of the gun is held in a special frame, which can turn with the gun vertically and horizontally. The frame and mount with recoil mechanisms are covered by a spherical armour cover, hanging off the mount.
According to order #079 issued on July 12th, 1943 by the People's Commissar of Tank Production and Commander of Armoured and Motorized Forces of the Red Army, a commission led by Lieutenant-General of Artillery, comrade V.E. Taranovich tested the following vehicles from July 25th to August 6th, 1943:
- SU-85-I with the 85 mm S-18-1 gun (TsAKB project with Uralmash modifications)
- SU-85-II with the D-5S-85 gun (factory #9 project)
- SU-85-IV with the 85 mm S-18 gun (TsAKB project)
- SU-122-III with the 122 mm D-5S-122 howitzer (factory #9 project)
The commission made the following conclusions following trials:
- All four vehicles passed mobility trials, but not gunnery trials.
- Considering that rapid adoption of an 85 mm gun by the Red Army is necessary, the commission considers the 85 mm SPG armed with the D-5S-85 gun designed by factory #9 and using the armoured self propelled chassis with a casemate that flares out above the entire width of the tracks.
- Factory #9 must make the necessary changes in the blueprints of the 85 mm D-5S-85 gun to correct defects of the gun and its mechanisms, established by the commission during trials and outlined in the "conclusions" section of the report."