Leather football helmets were introduced in 1893, after doctors warned that a blow to the head could be fatal or cause 'instant insanity.' Football helmets didn't really catch on until the 1920s, however, and weren't mandatory until the 1930s.
When most people think of classic vintage football helmets, they think of the leather padded style popular in the 1940s. The first logos were put on helmets in 1948, and leather was soon phased out. The helmets most valued by collectors today are the older leather ones, with the exception of more modern helmets worn (and/or signed) by famous players, and those with logos which are no longer in use or from teams that no longer exist.