In the late 40s, Timely Comics (predecessor to Marvel) got the notion to start producing female superheroes.  It came from the response Miss America got when they gave the character her own title.  Why she got her own title in the first place probably lies in publisher Martin Goodman hearing how well Wonder Woman and Mary Marvel were selling.  That was also about the time Harvey Comics gave Black Cat her own book, so Goodman may have sensed a trend.  So he flooded the market with female-oriented titles and characters: Millie The Model, Tessie The Typist, and other such titles, along with Miss America, Namora, Sun Girl and The Blonde Phantom.  Plus Golden Girl replaced Bucky in Captain America.

Unfortunately, sales on the others didn't match sales on Miss America.  Millie and Blonde Phantom stuck around a few years, but the others quickly disappeared.  It's too bad.  Blonde Phantom wasn't a bad book, although her costume is totally impractical.  And Namora was fairly interesting.  Timely and later Atlas didn't do a bad job with female heroes.  Venus under Bill Everett was very good, and Lorna The Jungle Girl had some nice stories by Don Rico and some nicer art by Werner Roth.  They just never stuck with them, just like Timely and Atlas rarely stuck with anything.  And the next interesting female Marvel would produce would be the Black Widow, and she was a villain in the Iron Man comic.