Thats cool. Is he a World War 1 era hero.....as opposed to current (when the book was published) ?
Early WWII looked a lot like WWI in many respects. There was incredibly rapid technological improvement once the war really started heating up. This book is cover dated March 1940, which means it was probably created in late 1939 or very early in 1940. Hitler invaded Poland in September 1939 so the cover image is probably more accurate that we would initially think.
At the start of WWII, the helmets were the leftovers from WWI basically - those ones with the wide brims and in peace time with countries bleeding money, no reason to replace them. Once the war efforts picked up, new equipment and uniforms came into play. During peace time, I guess it made sense to use what they had. Pretty quickly into the war, the style changed to what we are used to seeing. A lot of 1940 and 1941 books will show those covers. For instance, this cover had to be drawn immediately after Pearl Harbor and displaying the different American soldiers.