The SOMUA S.35 was developed in 1934 in response to demand for a cavalry tank that could withstand direct hits from contemporary anti-tank weapons. SOMUA was the company assigned to the challenge, and by spring 1935 they completed construction of the first prototype, designated the AC3. 

Unfortunately, early testing revealed some flaws with the first prototype, which SOMUA subsequently fixed and released in a pre-production batch of four vehicles (called the AC4). This first batch demonstrated satisfactory performance. In 1936, the AC4 modification of the vehicle was adopted as the standard medium tank for the cavalry units of the French army. Shortly thereafter, it received the official designation of SOMUA S.35. 

Production of the SOMUA S.35 began in 1936, with a goal of producing 600 vehicles. During production, however, financial restraints limited the number that could be produced. Of the initial 600 planned, only 450 were actually ordered and built for the French army. 

Following the outbreak of WW2, the S.35 was soon deployed to defend French soil against a German invasion. Because the tank was designed to fight contemporary armoured vehicles of other nations, it fared well against the onslaught of Panzer III’s and early modifications of the Panzer IV. 

In fact, in most battles where the S.35 clashed with German Panzers in engagement ranges of around 1000 meters, the S.35 was able to destroy the Panzers without taking any critical damage in return. 

Unfortunately, while the S.35 did reasonably well in combat, the mechanics servicing the vehicle had few kind words for it. Due to its cast hull and complex suspension design, the S.35 was notoriously difficult to service and maintain—so difficult that if the tank was damaged in combat, the crew was unlikely to be able to repair it on the battlefield.

While the S.35 saw some success in battle, its victories were short-lived. In June 1940, France surrendered and German forces occupied the northern part of the country, while the Vichy government was established in the south. Dozens of S.35s were captured, modified, and then deployed by the German and Italian forces. A handful of S.35s were also given to axis allies such as Hungary and Bulgaria. 

The S.35 was eventually decommissioned from active service at the end of hostilities in 1945.