The Somua S40 was an improved version of the Somua S35 that was under development just before the Fall of France. The S40 used the same basic layout as the S35, but with a 220hp Diesel engine, a new form of suspension and a welded turret.
The S40 was to use a new ARL 2C welded turret which was intended to speed up production by eliminating one of the key production bottlenecks with the S35. This carried the same 47mm gun as the APX 1 CE turret of the S35. The hull was similar to that designed for the SAu 40 self-propelled gun (see S35 article).
A contract for 50 S40 tanks was placed on 21 September 1939. It was expected to replace the S35 on the production line from vehicle 451, with the first 80 tanks using the old APX 1 CE turret and the 81st tank and onwards carrying the ARL 2C turret. Some of the hulls had been partly completed at the Fall of France, but none of the turrets were ready and neither was the new engine. A pilot with a wooden mock up of the new turret had been completed by April 1940, but never received the proper turret.
Interest in the S40 continued after the Fall of France. In November 1940 the Japanese government approached the Germans and French to investigate the possibility of ordering S40s for the Imperial Japanese Army.
In the spring of 1941 Admiral Darlan, then the prime minister in the Vichy Government, carried out discussions in Berlin in the hope of gaining concessions at home in return for increased German military rights in France's overseas colonies. These talks helped produce the Protocols of Paris of May 1941, an agreement for increased cooperation between Germany and Vichy France.
The Protocols include a plan to produce an improved version of the S40, using a new FCM welded turret. This would be a three man turret carrying an improved 47mm gun, with slightly sloped sides that merged smoothly into a fixed cupola. 800 tanks were to be built - 200 for French use in the colonies and 600 for Italy and France.
In February 1942, after the Japanese entry into the war, Vichy was allowed to arrange an outline agreement to build 250 improved S40 tanks for the Imperial Japanese Army, to be delivered 12-18 months after the contract was signed. Work on the tank had now moved from the Somua plant, which was at St Ouen in the occupied zone, to the FCM plant at Marseilles, in Vichy France. As work continued on the new design the Germans and Italians were kept informed.
By September 1942, when the Italian Armistice Commission was given the latest update, the plan was to build 135 S40s for three Vichy squadron groups in North Africa, 250 for Japan and as many as Germany and Italy wanted. Two turrets were under consideration - the improved three man turret armed with the 47mm SA37 gun or a less advanced two-man turret that would have carried the older 47mm SA35 gun.
All of these plans were ended by Operation Torch, the Allied invasion of North Africa. In response the Germans occupied Vichy France, disbanded the French armistice army and ended all open tank development.