As successful as the T-26 Light Tank had become for the Soviet Army in the early-to-mid-1930s, engineers were constantly attempting to improve the design. This ultimately led to a series of test and pilot vehicles which largely failed to become greater than its progenitor. One such project became the "T-46" (detailed elsewhere on this site) which entailed the T-26 framework mated to the American Christie suspension system (a departure from the T-26's original wheeled bogie arrangement). Tests proved the vehicle sound but cost was a key factor in its demise (a T-28 Medium Tank of greater power and tactical value could be had for the same price). Another entry following on the heels of the T-26's success during the period became the "T-25" - also known as the "STZ-25".
Once again, mobility was at the heart of this venture and development work began in the middle part of the 1930s leading to the pilot form being revealed in 1938. The work was handled by the Stalingrad Tractor Works and, once again, the Christie suspension system was at the heart of the T-26 offshoot project. The running gear involved four road wheels to a hull side with the drive sprocket at front and the track idler at rear with just one track-return roller being used. Engineers retained the internal running gear components as well as most of the original hull and turret component from the existing T-26 design with the exception being that the forward hull superstructure was modified to improve basic ballistics protection from head-on attacks.
Armament remained 1 x 45mm 20K L-46 main gun in the turret with local defense provided by 1 x 7.62mm FT machine gun in a co-axial mounting. Drive power was from a single GAZ-T-26 Otto 6-cylinder gasoline-fueled engine developing 95 horsepower to the track-and-wheel arrangement. Road speeds could reach 28kmh and range was out to 200 kilometers. Combat weight reached 11.7 tonnes with dimensions including an overall length of 4.85 meters, a beam of 2.68 meters and a height of 2.37 meters. The crew remained three (driver, commander and gunner). Armor protection reached 24mm at its maximum.
Like the BT series of Fast Tanks (detailed elsewhere on this site), the T-25 would have the ability to run on its road wheels without its tracks. This improved road speeds and operational range (in prepared roads) considerably, the vehicle now ranging out to 400 kilometers.
Testing on the T-25 ensued in 1939 but the mechanical system proved unreliable under conditions and, again, serial manufacture cost was became an issue going forward. Engineers attempted to remedy this with the development of the simplified "STZ-35" / "ST-35" design but even this tank evolved no further.
With the arrival of World War 2 in September of 1939, such side projects as the T-25 took a backseat to more pressing matters for the Red Army.