In 1961, Willis produced the first of his Artist's Sketch Pad calendars for Brown and Bigelow and these continued into the seventies; the last survivors of the illustrated classic pin up period.
Willis' pictures had some distinctive features which make them easy to identify. Some of the distinctive features belong to his wife Pat, who modelled for many of his paintings. The use of unfinished sketches on the final painting were another feature of his work. Quite often he would pose his model holding a cup of tea.
Unlike many other pin up artists, he also occasionally depicted a girl's fluff which really only Vargas did in his later work for Playboy in the mid seventies.
Willis' women were in complete contrast to someone like Gil Elvgren's innocent cuties. "The Willis Girl", as she was christened by Esquire (they were always desperate to find another "Varga Girl") looked a little older, more sophisticated, richer and a lot more worldly wise than her rivals.