Some sources have called these Heavy tanks which would probably be a better classification for them, but I'll leave them listed as Medium tanks for now.
In 1921 General J.E. Estienne (pioneer of French tank arm) of the Section Technique des Chars de Combat initiated requirements for a 15 ton tank with a 75 mm or 47 mm in the hull. Tracteur 30 was the codename for the design.
Schneider-Renault, FAMH, and FCM submitted 3 prototypes. In 1926 the Section Technique decided to take the suspension from the FCM and the mechanical features from the Schneider-Renault. The ARL (Artelier de Construction de Rueil) supervised the design and the first Char B was finished in 1929. Early prototypes were developed over the years and in April 1935, 35 / 40 Char B1s were delivered.
In March 1935, the Germans re-occupied the Rhineland. This caused the French to start to re-arm. 40 Char B1s were ordered in April 1935 with 60 mm of armor. These became the Char B1 bis.
Were nicknamed Kolosse by the Germans when the encountered them.
There was a compressed air and electric starting system for the engine.
The fuel tanks were self sealing.
The suspension was based on the Holt tractor. On each side there were three assemblies with four bogies mounted in pairs. There were three independently mounted bogies in the front and one in the rear. The idler wheel in front was spring mounted as well.
There was a Naeder hydrostatic system for steering, when combined with a double differential unit, allowed for very precise turning that was necessary for aiming the gun.
The driver sat in the left front, drove the tank, and fired the 75 mm gun and machine gun in the hull. The commander had to aim, load, and fire the turret guns as well as command the tank. The loader has to pass ammunition to the commander and load the 75 mm gun in the hull. The radio operator was near the turret. The emergency exits for the crew were a door in the right side of the hull, hatch over the driver, hatch in the right rear of the turret, hatch in the floor, and a hatch in the roof of the engine compartment.
The 7.5 mm Chatellerault machine gun was on the right of the hull, and could be aimed by the driver or the commander.
The 75 mm gun was aimed by traversing the whole tank.
There was a Luchard air compressor that blew the smoke and fumes from the gun out the barrel in a similar fashion to naval gun turrets. The whole tank had to be turned to move the 75 mm gun into firing line.
The hull had two parts which were separated by a fireproof bulkhead. The rear held the engine, and the front the crew.