From the S35, SOMUA developed in 1937 the SAu 40 self-propelled 75 mm gun.
Its drivetrain featured an extra wheel to improve off-road capabilities, and the hull was wider.
 Only one prototype was built in this configuration; it fought in June, probably together with a small number of mass-production vehicles, fitted with the powerful 47 mm SA 37 gun: 72 units had been ordered on 1 May 1940.

In order to address some of the shortcomings of the S35, SOMUA presented the improved AC5 type in 1939.
Based on the SAu 40 chassis and its Char G1-project but with the original width, this SOMUA S40 had a welded ARL 2C turret and redesigned cast superstructure, both to lower production costs and to improve protection standards as the cast sections, delivered by eighteen subcontractors, sometimes were of inferior quality.
The new suspension strongly improved the climbing capacity, of which the Cavalry had officially complained in November 1938; its greater weight was compensated by lowering the hull height by fourteen centimetres, causing a weight gain of four hundred kilogrammes.
In contrast, the engine deck was raised to fit an improved 230 hp engine, increasing maximum speed to 45 km/h, although the new engine was not yet available in the summer of 1940.
The armament and general nominal armour base would remain the same, but first steps to improve these, which might have naturally have evolved into a "S 41", had already been taken in the spring of 1940, when plans were made for a 60 mm welded ARL turret.
A first order was on 21 September 1939 made for fifty vehicles, and it was intended to become the main production type, superseding the S 35 from the 451st vehicle with total orders having reached four hundred hulls, but none were completed at the time of the German invasion; the first vehicles were planned to be produced in July; hull sections had already been cast since November 1939.
Of the first 160 vehicles eighty had been planned to be made of an intermediate type, with the old turret.

After the armistice plans were developed to resume production, partly for the benefit of the Axis powers.
On 28 May 1941, the German ambassador in France Otto Abetz concluded an agreement with the French government, the Protocols of Paris.
These included the intention to produce eight hundred SOMUA S40s, two hundred for France itself and six hundred for Germany and Italy.
However, Hitler, suspicious of a French rearmament, declined ratifying the agreement.

In November 1940, the Japanese government had requested Germany to allow production for Japan.
When Japan became a belligerent through the Attack on Pearl Harbor, on 9 February 1942 it was decided that France would produce 250 SOMUA S40s for the Japanese Imperial Army, the first to be delivered in twelve months time, production having to reach a peak of eight vehicles per month in eighteen months.
The events of November 1942 precluded both production and delivery.

In early 1942, France was finally given permission to reequip its forces with SOMUA S40s, now that the type had become obsolete and the Germans had lost interest.
Two versions were considered on 24 April 1942, both fitted with the larger FCM-turret, originally developed for the Char G1.
The first would have been armed with the SA 35 gun, operated by a two-man turret crew, the second with the longer SA 37 gun placed in a three-man turret.
France foresaw a production of 135 vehicles, to equip three squadrons of 45 each, but the preparations were halted in November 1942 when the whole of France was occupied.
However, a clandestine development continued, of a SARL 42, fitted with the ARL 3 turret and a 75 mm L/32 or L/44 gun using an optical range finder.
To limit weight, its side armour was to be diminished to thirty millimetres.

In 1945 it was proposed to build a tank destroyer by refitting existing S 35 chassis with a superstructure for the British 76.2 mm 17-pounder gun.
From the S35, SOMUA developed in 1937 the SAu 40 self-propelled 75 mm gun. Its drivetrain featured an extra wheel to improve off-road capabilities, and the hull was wider.  Only one prototype was built in this configuration; it fought in June, probably together with a small number of mass-production vehicles, fitted with the powerful 47 mm SA 37 gun: 72 units had been ordered on 1 May 1940.
In order to address some of the shortcomings of the S35, SOMUA presented the improved AC5 type in 1939. Based on the SAu 40 chassis and its Char G1-project but with the original width, this SOMUA S40 had a welded ARL 2C turret and redesigned cast superstructure, both to lower production costs and to improve protection standards as the cast sections, delivered by eighteen subcontractors, sometimes were of inferior quality. The new suspension strongly improved the climbing capacity, of which the Cavalry had officially complained in November 1938; its greater weight was compensated by lowering the hull height by fourteen centimetres, causing a weight gain of four hundred kilogrammes. In contrast, the engine deck was raised to fit an improved 230 hp engine, increasing maximum speed to 45 km/h, although the new engine was not yet available in the summer of 1940. The armament and general nominal armour base would remain the same, but first steps to improve these, which might have naturally have evolved into a "S 41", had already been taken in the spring of 1940, when plans were made for a 60 mm welded ARL turret. A first order was on 21 September 1939 made for fifty vehicles, and it was intended to become the main production type, superseding the S 35 from the 451st vehicle with total orders having reached four hundred hulls, but none were completed at the time of the German invasion; the first vehicles were planned to be produced in July; hull sections had already been cast since November 1939. Of the first 160 vehicles eighty had been planned to be made of an intermediate type, with the old turret.
After the armistice plans were developed to resume production, partly for the benefit of the Axis powers. On 28 May 1941, the German ambassador in France Otto Abetz concluded an agreement with the French government, the Protocols of Paris. These included the intention to produce eight hundred SOMUA S40s, two hundred for France itself and six hundred for Germany and Italy. However, Hitler, suspicious of a French rearmament, declined ratifying the agreement.
In November 1940, the Japanese government had requested Germany to allow production for Japan. When Japan became a belligerent through the Attack on Pearl Harbor, on 9 February 1942 it was decided that France would produce 250 SOMUA S40s for the Japanese Imperial Army, the first to be delivered in twelve months time, production having to reach a peak of eight vehicles per month in eighteen months. The events of November 1942 precluded both production and delivery.
In early 1942, France was finally given permission to reequip its forces with SOMUA S40s, now that the type had become obsolete and the Germans had lost interest. Two versions were considered on 24 April 1942, both fitted with the larger FCM-turret, originally developed for the Char G1. The first would have been armed with the SA 35 gun, operated by a two-man turret crew, the second with the longer SA 37 gun placed in a three-man turret. France foresaw a production of 135 vehicles, to equip three squadrons of 45 each, but the preparations were halted in November 1942 when the whole of France was occupied. However, a clandestine development continued, of a SARL 42, fitted with the ARL 3 turret and a 75 mm L/32 or L/44 gun using an optical range finder. To limit weight, its side armour was to be diminished to thirty millimetres.
In 1945 it was proposed to build a tank destroyer by refitting existing S 35 chassis with a superstructure for the British 76.2 mm 17-pounder gun.
SOMUA S35