Hitler certainly did not personally design the German feldbluse. The Reichsheer, the Weimar army, introduced the new uniform in 1933, the year Hitler came to power, but the design had been in the works for two years by that time. It's really, really hard to point to an individual responsible, as this was a uniform designed by committee; I certainly cannot locate any of the individuals responsible. In any case, the so-called M40 (this is a collectors' designation; the Germans did not differentiate it from earlier or later models) was a manufacturers' wartime simplification of the earlier model feldbluse, not an official submodel. German feldblusen became gradually less and less complex as the war went on: first the bottle green collar was lost, the uniform cloth became increasingly poor quality (more than 50% artificial fibers by the last two years of the war) and an additional button had to be added to make up for the loss of tensile strength, the pockets became markedly less complex, and then, finally, it was wholly redesigned as the short jacket M44.

As far as the so-called M35 stahlhelm? It seems to have been designed by Eisenhuttenwerke of Thale in response to a Reichsheer contract. EK would become the major producer of stahlhelme for the war, leading to the ubiquitous EK stamp on the inside rim of German helmets.