Throughout the 1930s, aircraft development was evolving by leaps and bounds across the world. As such, air defense forces of the world needed a similar evolution to keep pace with many of the current ground-based offerings having become outmoded by faster and higher flying aircraft. The Soviet Union was one such military force that had a vast amount airspace to concern itself with should she ever be invaded. As opposed to developing an entirely new system from scratch for the Red Army, Soviet authorities elected to utilize the existing - and successful - 76.2mm (3-inch) Model 1938 air defense gun as the basis for a new, more potent and capable weapon.
Design and development work soon yielded the 85mm (3.346-inch) Model 1939 air defense gun. Similar in scope and operation to her predecessor, the Model 1939 was essentially nothing more than an enlarged and upgunned version of the original 76.2mm system. A key differentiating feature was a multi-baffled muzzle brake fitted to the business end of the barrel, a physical feature that the Model 1938 clearly lacked. An forward facing armored shield could be affixed to the gun mount as an option for added crew protection during the heat of battle but offered little true protection overall. Like the fabled "88" anti-aircraft guns of the German Army, the Red Army also saw double value in their M1939 and attention in design was also given to utilizing it as an anti-tank defense weapon should the need arise.
The 85mm gun mount sat on a four-wheeled traveling carriage for maximum portability. The carriage could be transported into position by any number of towing vehicles then in service with the Red Army, from trucks and jeeps to light tanks. The seven man crew could also pushed the gun into a precise position once at a target area. The gun system itself was fully positional apart from the carriage and could attain an upwards attack angle to target incoming enemy aircraft or a leveled attack angle to counter oncoming enemy vehicles or infantry. Elevation limits were stated at +82 and -2 degrees while traverse was a full 360-degrees. When in transport, the gun was leveled and locked into place with an inverted "vee" type bracket. The weapon weighed in at 9,303lbs when traveling and 6,739lbs when made ready to fire. Overall, the system measured in at over 23 feet when in transit while the barrel itself was over 15 feet long. Along with the standard flak in-air exploding shrapnel rounds, M1939 crews were also issued with a supply of anti-tank projectiles.
By the time the M1939 was entering production, the facilities at Kaliningrad outside of Moscow came under direct threat from the surprise German invasion of the Soviet Union in 1941. As such, the Soviets took to moving the manufacturing equipment out of the area and relocating them to the safety of the Ural mountains to the East. Within time, the facilities were set in place and production of the new M1939 soon got underway. When the delivery numbers dictated, the M1939 was officially recognized as the standard anti-aircraft defense gun of the Soviet Army.
The M1939 proved an excellent gun platform throughout the war. The system was delivered to the active fronts and utilized in defense of strategic positions. The weapon proved quite valuable in the war for both sides, so much so in fact, that even the Germans considered its capture as something of a war trophy. Enough M1939s and available 85mm ammunition were captured by the advancing German Army that many M1939s were reconstituted back into service for the Wehrmacht under the designation of 8.5cm Flak M.39(r) and used extensively alongside their excellent 88mm guns with powerful results. Additionally, other captured M1939s were delivered back to Germany, rebored to the standard German Army 88mm caliber and setup in defense of the homeland under the new designation of 8.8cm Flak M.39(r).
The M1939 was replaced in its role with the Soviet Army by the more powerful 85mm Model 1944 series (KS-18) before the end of the war. The Model 1944 was more or less the same gun system as the M1939 before it, however, its projectiles were issued with a propellant charge of increased power, making the M1944 a better alternative to the M1939. The 85mm gun was further modified for use in the SU-85 tank destroyer and many M1939/M1944 guns were still in use throughout the air war over Vietnam of the late 1960s and early 1970s. The M1939 can still be found in operational service with a few countries around the globe, mostly former Soviet satellite states and Cold War allies.