Before Iron Man launched the Marvel cinematic universe, the studio was developing a Black Widow movie with Lionsgate that was going to be released before Iron Man, but the studio killed.
Spinoff Online offers up the details on what happened, explaining that back in 2004 X-Men screenwriter David Hayter wrote the script for the Black Widow film, and it was supposed to be his directorial debut. Once he completed the screenplay, though, Lionsgate decided to pull out and didn't want to make the film anymore. Hayter explained why,
"Unfortunately, as I was coming up on the final draft, a number of female vigilante movies came out. We had Tomb Raider and Kill Bill, which were the ones that worked, but then we had BloodRayne and Ultraviolet and Aeon Flux. Aeon Flux didn't open well, and three days after it opened, the studio said, 'We don't think it's time to do this movie.' I accepted their logic in terms of the saturation of the marketplace, but it was pretty painful."
So we can blame several other female-led films that bombed at the box-office for the demise of Black Widow. Just because those movies didn't do well didn't mean Black Widow would have failed, though. Tomb Raider and Kill Bill didn't! After Lionsgate dropped out, Marvel shopped the movie around to other studios, but none of them were interested. Once they got the full rights to the character back, they introduced her into the Marvel cinematic universe in Iron Man 2.
I don't know what that version of Black Widow would have been like, but I really love what they've done with the character in the universe they've built, and I'm sure one day Marvel will do a standalone Black Widow movie. Until that day, I will enjoy seeing the character kick ass in the ensemble films that she plays a part in.