Like other powers of the interwar period, the French were prudent in modernizing their armored corps and part of the upgrading involved adoption of the Renault R35 Light Infantry Tank (detailed elsewhere on this site) in 1935. The R35 was a well-protected track-and-wheel combat system but, it was soon found, lacked performance and its short-barreled 37mm main gun did not match up well against more stout armor by the time of the German invasion of May 1940. Additionally, the suspension system was poorer than anticipated and several projects were enacted by various parties to help improve the line some.
Leading up to World War 2 (1939-1945), a new suspension design from AMX was selected for installation on existing R35 tanks already in service. Testing of the new arrangement ended in December of 1938 and the decision was made in February of 1939 to upgrade the series. The modified tanks would be recognized as "Char Leger Modele 1935 R Modifie 1939" but history better remembers the variant as the Renault "R40" (though this was not an official type designation by the French Army) due to its many instituted changes.
At the outbreak of war, all of these plans changed and the modifications were pressed onto new-built R40 tanks instead. The R40 forms appeared from the 1,541 modified R35 tank onward. While still on the lines, the tanks were also reworked to accept the 37mm SA38 Long 35 cannon in its long-barreled form with a new cast turret. A communications set was also fitted - a feature sorely lacking on original R35s. A lighter, welded turret by the FCM company was also scheduled to be introduce - though this would not happen until late-1940 and by then the German conquest of France would be complete.
During the invasion of France the R40 stocked at least two French battalions and a single Polish brigade fighting in France. All of these were either lost in battle or claimed by the conquering Germans and their fate sealed. None managed to survive the war and it is believed that only about 130 of the type were completed before the German victory - while a few dozen more hulls lay awaiting their turrets which never arrived.