As early as 1932 (and, for all I know, before that) the cinema has offered us glimpses of male stars in towels. This shot is of Joel McCrea in The Sport Parade. He's on the left with pal William Gargan on the right.
here a link to the magazine article and more photos (be sure to click “continue” at the bottom of the first page.)
here and here.
Of course, years before, Newman had demonstrated a far more muscular and lean physique when he spent a considerable amount of time in a towel during 1963's The Prize.