The Collaborative International Dictionary of English v.0.48:

Benefactress \Ben`e*fac"tress\, n.
   A woman who confers a benefit.
   [1913 Webster]

         His benefactress blushes at the deed.    --Cowper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

benefactress
    n 1: a woman benefactor