The Black Widow's first appearances were as a recurring, non-costumed, Russian-spy antagonist in the feature "Iron Man", beginning in Tales of Suspense #52 (April 1964).

Five issues later, she recruits the besotted costumed archer and later superhero Hawkeye to her cause. Her government later supplies her with her first Black Widow costume and high-tech weaponry, but she eventually defects to the United States after appearing, temporarily brainwashed against the U.S., in the superhero-team series The Avengers #29 (July 1966). The Widow later becomes a recurring ally of the team before officially becoming its sixteenth member many years later.

Source - Wikipedia