The SOMUA S35 was a French cavalry tank of the Second World War. Built from 1936 until 1940 to equip the armoured divisions of the Cavalry, it was for its time a relatively agile medium-weight tank, superior in armour and armament to its French and foreign competitors, such as the contemporary versions of the German Panzer III medium tank. It was constructed from well-sloped, mainly cast, armour sections, that however made it expensive to produce and time-consuming to maintain.

During the German invasion of May 1940, the SOMUA S35 proved itself to be a tactically effective type, but this was negated by the French command's strategic mistakes in deploying their Cavalry armoured divisions. After the defeat of France in June 1940, limiting production to a total of about 440, captured SOMUA S35s were used by the Axis powers, some of them on the Eastern Front. A derived type, the SOMUA S40, with an improved suspension, lowered hull cast and welded turret armour, had been planned to replace the original version on the production lines in July 1940. Agreements to produce this improved type for the benefit of Vichy France, Germany and Japan, ultimately did not lead to any manufacture.
The design of the SOMUA S35 comes from the changed specifications of 26 June 1934 for an Automitrailleuse de Combat (AMC) issued for cavalry use.[1] These called for a much heavier design than had been originally specified in 1931. The new type had to be immune to contemporary anti-tank guns.
The prototype, with the name AC3, was ready on 14 April 1935. It was tested from 4 July until 2 August 1935. Then a pre-series of four was produced of an improved type, the AC4, to be tested until 27 January 1938. These first vehicles had the standard APX1 turret, fitted with the short 47 mm SA 34 gun. On 25 March 1936, the AC4 design was accepted as the standard medium tank of the French Cavalry with the official name Automitrailleuse de Combat modèle 1935 S (AMC 1935 S), when a first order for fifty was made. The tank was then more commonly known as SOMUA S35 (S for SOMUA and 35 from 1935, the year of introduction); today the even shorter abbreviation "S35" is most often used, in English sources usually with a hyphen: "S-35".
The mass-production vehicles would have the longer SA 35 gun. Originally, a total production of six hundred was planned to provide each of the three Cavalry armoured divisions with two hundred tanks. Later, budgetary restraints led to a more gradual and limited procurement. In 1936 a second order was made of fifty, followed by a hundred in 1937, and two orders of 125 each in 1938, resulting in total prewar orders of 450 units.
The hull and turret were castings with a maximum thickness of 47 mm and 40 mm respectively - the former of four sections, bolted together: two longitudinal plates formed the bottom; the superstructure was divided in a front and back section.
The turret was a variant of the APX 1 as used on the Char B1: the APX 1 CE (chemin élargi) with a larger 1,130 mm (44 in) as against 1,022 mm (40.2 in)) turret ring, allowing the radio operator to assist the commander in loading the gun from an ammunition stock of 118 shells (90 AP, 28 HE) and 2,250 machine gun rounds. As with the B1, the commander was expected to direct the tank while also loading, aiming, and firing the 47 mm SA 35 main gun - although at least the radio duty could be left to another crew member. Radios were planned to be part of the standard equipment of S35s. In practice the platoon commander had an ER (émetteur-récepteur) 29 set for communications with a higher command level, but a shortage of the short range ER28 sets for communication within the platoon meant that the other four tanks of the platoon were never fitted with any form of radio, although in some units all tanks had antennas: the programme to fit the sets themselves was postponed until the summer of 1940 and thus overtaken by events.
The suspension was designed by Eugène Brillié, the same man who had developed the first French tank, the Schneider CA1. He had worked with the Czech Škoda company and based his design on that of the LT35: eight road wheels paired on four bogies with leaf springs and an equally large tension wheel. The first fifty vehicles had tracks consisting of 144 track links, each link with a length of 75 mm; later vehicles had 103 links of 105 mm length.
The engine was in the rear of the hull side by side with two self-sealing fuel tanks, of 100 and 410 litres respectively, separated from the fighting compartment by a firewall bulkhead. The nominally 200 hp engine, designed by Javier-Sabin, drew fuel from the smaller tank, which was itself automatically replenished from the larger one. Inexperienced crews sometimes made the mistake of only filling the smaller tank.
Engine and suspension maintenance was difficult and time-consuming, due to a poor accessibility, though this was improved in later vehicles.
S 35 had a fire extinguishing system methyl bromide.