The SAu 40 self-propelled gun was developed by the SOMUA company between 1935 and 1937 as part of the same order the French cavalry placed for the S35 tank.
The two tanks combined were intended to equip light mechanized divisions of the French army and support the larger “battle tanks” (chars de bataille). The vehicle inherited the S35’s characteristic chassis with a redesigned front section of the hull to accommodate the 75mm APX cannon. In addition, the rotating turret on top of the fighting compartment was redesigned to a commander’s cupola and moved to the side of the vehicle, but the turret’s 47mm cannon was replaced with a single 7.5mm machine gun.
The body of the SAu 40 was ready by 1937, but it couldn’t be immediately completed. Because the cannon was not yet ready to be installed on the chassis, SOMUA engineers had to wait nearly 2 years before the cannon was finally made available. It wasn’t until 1939 that the fully assembled prototype was completed, and orders for mass production were received in October of the same year.
Whether or not any additional vehicles other than the prototype were built or if the vehicle took part in active fighting is unknown.