The Char B1 was a specialised break-through vehicle, originally conceived as a self-propelled gun with a 75 mm howitzer in the hull; later a 47 mm gun in a turret was added, to allow it to function also as a Char de Bataille, a "battle tank" fighting enemy armour, equipping the armoured divisions of the Infantry Arm.

The specifications included: a maximum weight of thirteen tonnes; a maximum armour thickness of 25 millimetres; a hull as low as possible to enable the gun to fire into the vision slits of bunkers; a small machine gun turret to fend off enemy infantry attacks, at the same time serving as an observation post for the commander and a crew of at most three men.

He used his position as Inspector-General of the Tanks to enforce the so-called "Estienne accord" on the industrialists, ordering them to "reach a mutual understanding, free from any spirit of industrial competition".

On these conditions four projects were started in 1921: two by a cooperation between Renault and Schneider: the SRA and the SRB, one by Forges et Aciéries de la Marine et d'Homécourt (FAMH), more commonly known as "Saint Chamond" from its location, and the last by Forges et Chantiers de la Méditerranée (FCM), the FCM 21.

A fifth producer, Delaunay-Belleville, whose project (an improved Renault FT) had been rejected beforehand, would be allowed to make 83 tanks; the remaining 167 would be allotted at the discretion of the French State.

All projects used a three-man crew but differed considerably in size, form and the solution chosen to laterally point the gun.

It was steered by an epicyclical transmission combined with hydraulically reinforced brake disks, which failed to provide the desired precision during tests.

The 75 mm howitzer was placed in the middle of the hull and steered by providing each snake track with its own hydraulic Jeanny transmission.

In March 1925, Estienne decided to base the future production type on the SRB, as regarded the general form and mechanical parts.

However, it would be fitted with the 75 mm gun, a Holt-track to be developed by FCM, which company had completed a special research programme aimed at optimising weight distribution, and the FAMH-suspension (later this would again be discarded).

Estienne also had some special requirements: a track tension wheel should be fitted, adjustable from the inside, and a small gangway from the fighting room should improve the accessibility of the engine compartment.

On 27 January 1926, it was decided to build three prototypes of what was provisionally called a Tracteur 30, a final design by engineer Alleaume of the Schneider company, cooperating with the Section Technique des Chars de Combat (STCC).

The hull of first Renault vehicle, made of soft boiler plate instead of armour steel to simplify changes, was finished apart from the armament in January 1929; it was delivered in March.

Technical aspects were not forgotten during these tests and it was established they could attain an average road speed of 19 km/h, cross a trench 2.4 m (7.9 ft) wide, and wade through a 105 cm (41 in) deep stream.

The similarity resulted partly from the fact that the Char B1 was a specialised offensive weapon, a break-through tank optimised for punching a hole into strong defensive entrenchments, so it was designed with good trench-crossing capabilities.

Another explanation of the similarity to the British Mark I lies in the Char B1's original specification to create a self-propelled gun able to destroy enemy infantry and artillery.

The main weapon of the tank was its 75 mm howitzer, and the entire design of the vehicle was directed to making this gun as effective as possible.

This is in contrast with the contemporary German, British and to a lesser extent[a] Soviet policy to use two or three-man turret crews, in which these duties were divided amongst several men.

This had a poor anti-tank capability: the thirty Armour Piercing High Explosive (APHE) rounds among the fifty the tank carried had a maximum penetration of about 25 mm.

The 75 mm L/17.1 gun, able to fire both a High Explosive and the APHE Obus de rupture Modèle 1910M round, had a limited traverse of only one degree to the left or the right.

Development of the Char B1 ter was started at the same time as production funds were given for the bis with the intention of providing a tank armoured to 75mm.

[6] A design with sloped and welded 70 mm armour, weighing 36.6 tonnes and powered by a 350 hp (260 kW) engine was meant to replace the B1 bis to accelerate mass production, a change first intended for the summer of 1940 and later postponed to March 1941.

[6] Cost was reduced by omitting the complex Neader transmission and giving the hull gun a traverse of five degrees to each side instead.

The regular divisions destroyed quite a few German tanks but lacked enough organic infantry and artillery to function as an effective mobile reserve.

Other differences to the French Army version included a jack and hoist arm carried outside the tank, with additional German equipment mounted on fenders.

In German service, the tank saw action in the Balkans campaign and the Eastern Front, initially during Operation Barbarossa, the flamethrower version from 1942 onwards.

[8] Some Char B turrets were removed and installed on German bunkers defending Normandy beaches at the time of D-Day (6 June 1944).

[10] They were stationed in Arnhem during Operation Market Garden, losing six tanks to anti-tank weaponry when they were sent to attack the Oosterbeek perimeter on 20–21 September 1944.

[15] The Italian armour historian Nicola Pignato in contrast stated in 1989 that some twenty B1 bis, in various stages of preparation and construction, along with a single 36-ton B1 ter prototype were directly recovered from the FCM factory, of which an unknown number were to be destined for Italy.

[23] The last surviving Char B1 can be seen at the Association pour la Sauvegarde du Patrimoine Historique et Militaire (ASPHM), near Strasbourg, in France.