In the words of Doctor Who creator Sydney Newman:

At a later stage Doctor Who should be metamorphosed into a woman. Don’t you agree that this is considerably more worthy of the BBC than Doctor Who’s presently largely socially valueless, escapist schlock? … This requires some considerable thought – mainly because I want to avoid a flashy, Hollywood Wonder Woman, because this kind of heroine with no flaws is a bore.

Thanks to Den of Geek: Doctor Who Creator Sydney Newman Had Plans for a Female Doctor.