They were not modified football helmets, but the resemblance is not coincidental. First, they perform similar functions...protecting the head from impact. Second, they were designed by the Rawlings Sporting Goods Company, who also made the football helmets.

The need for head protection becomes immediately obvious to anyone who has ridden in a tank. Then-Captain George S. Patton, after observing French and British tanks, recommended in a memo in December of 1917 that "Leather helmets like those worn by football players or aviators, but without ear pieces, must be provided for the crews to prevent their being knocked unconscious when going over rough ground." Patton may have procured such a helmet locally for US tank crews in France.

This lead to the 1919 pattern "Helmet, Tank", which was more of a leather cap. Not until 1936 did the cavalry branch seek a replacement, eventually choosing a design submitted by Rawlings. This design had a hard shell made of resin impregnated paper laminate and soft leather cheek pieces (The cavalry was not allowed to have tanks, only the infantry, so their tanks were called "combat cars"). The Infantry Board also requested submissions from Rawlings for a replacement helmet, resulting in their own version, with a more prominent foam rubber bumper ring around the hard shell, and provisions for retaining radio headphones in the ear pieces. Rawlings produced most of these, but they were also produced by the Sears Saddlery Company.

The Cavalry Type only came in 3 sizes, and lacked accommodations for the radio headphones. The Infantry Type was deemed too heavy and bulky, as the rubber bumper interfered with the gunsights. In May, 1941, five prototypes were tested, from Rawlings, Clark, Goldsmith, Riddell, and Mine Safety Helmet (The first four being football helmet makers). The Rawlings design was chosen, with some modifications based on the Clark design.

US Army Tank Crew Helmets, Larry Munnikhuysen, September 1, 2017

Capt. George S. Patton, Jr., "Memo To: The Chief of the Tank Service,” 12 December 1917

“A Tanker’s Uniform for a Tanker’s Duties,” Research Report, Armored School, Fort Knox, Kentucky, 1951-1952.