Logos on the side of helmets have been the norm since Fred Gehrke painted horns on the Los Angeles Rams' helmets in 1948, but logos on the front and back of helmets preceded the Rams' horns. The University of Chicago had their Wishbone C on the back of their helmets in 1921 and 1922. Logos on the back never became popular, though we'll see below that two other Big Ten teams had them about forty years later.
Besides Chicago, the earliest logos were small letters carved in the front padding of winged helmets, though only a handful of teams had them, including Purdue and Michigan State in the mid-1930s.