Steve, WWI tanks are manufactured by Emhar, amongst other brands. They have the Whippet, the Mk IV Female and Male, plus the Tadpole, the A7V and are soon releasing the MkV.
Tauro Models also had the German A7V. As to the early French tanks, the Renault FT is also available in that scale (but I can not remember who by). The St Chamond and the Schneider are available in resin.
The Char B1/B1bis was not used at all during WWI, but only appeared before WWII. At the time, it was possibly the most powerful and most protected AFV in the world, but it's achille's tendon was the fact the tank commander was also responsible for firing the turret gun, limiting his time in doing other vital tasks... As far as I know, there are sadly no 'mainstream' kits opf this vehicle in 1/35, only resin kits that are rather expensive...
The British Mk I Male and MkII Male of WWI had long guns siticking out of the side sponsons and those often geot stuck in the ground or bent. That was the main visible modification made to the tank to turn it into the MkIV.