As the Marvel Cinematic Universe's top (and for a long while only) female superhero, a lot has been made over the fact that Black Widow hasn't had her own solo movie yet.
Whenever Marvel announces a new movie, there is always a legion of fans demanding that Natasha Romanoff get a big screen outing all to herself. Marvel have been pretty cagey about announcing anything though.
What Marvel probably don't want adding fuel to the fire then is that Black Widow actually almost had her own movie well before any other Avenger.
In 2004, before Marvel got the rights to the character back, Lionsgate had a Black Widow origin movie in development. The draft script involved Natasha being trained as a child assassin and given the Black Widow super-soldier serum before defecting from Russia to the US. It was written by David Hayter who had the twin qualifications of being a screenwriter for the X-Men films and being the voice of one of the world's other favourite super-spies Solid Snake.
The film was cancelled though when Ultraviolet and Aeon Flux bombed. In a typical case of executives being executives, they decided it was due to people not wanting female-led action films rather than people not wanting action films that weren't terrible.