Tales of Suspense
With Tales of Suspense #52 (April, 1964 by Stan Lee and Don Heck), the Black Widow was introduced into the world of Iron Man!
Natasha Romanoff was sent along with Boris Turgenev to kill Anton Vanko (the original Crimson Dynamo), as well as Tony Stark and Iron Man (not knowing they were one and the same.
Boris ended up becoming the next Crimson Dynamo for a time, and Natasha had established herself as a femme fatale in Iron Man's life....though not quite as active Black Widow has been seen!
...but still not being able to defeat him!
While quite stylish, an evening gown, gloves and fur are not the best way to battle a man in armor! Next time around (in Tales of Suspense #57, September, 1964 by Stan Lee and Don Heck), Natasha uses her feminine wiles on the battling bowman known as Hawkeye, taking a misunderstanding he had with the police in his introduction to turn Hawkeye into a criminal, and set Hawkeye and Iron Man at odds!
Natasha was there again, setting Hawkeye against Iron Man in Tales of Suspense #60 (December, 1964 by Stan Lee and Don Heck), and things just got worse for Hawkeye, but it did seem Clint Barton and Natasha were getting closer for real...so much so, that when she appeared next in Tales of Suspense #64 (April, 1965, by Stan Lee and Don Heck), Natasha had a costumed identity of her own...but she didn't appear to be as much help as one would have thought, as she had been injured (enough that she wasn't as close to Hawkeye in their next appearance in a flashback in Avengers #16...and then Hawkeye joined the Avengers!
Black Widow didn't appear in Avengers until Avengers #29...
....and didn't join the team until much later..
..though she did hang around with Earth's Mightiest Heroes for a time, and met Hercules, as well as the team finding her husband, the Red Guardian...the Russian version of Captain America).
It took a while of working on and off with the Avengers, and Natasha leaving Clint Barton (now as Goliath), and a chance meeting with Spider-Man in Amazing Spider-Man #86 (July, 1970, by Stan Lee and John Romita) for Natasha to get a new uniform, and start her own series in Amazing Adventures (that ran for 8 issues, from Amazing Adventures #1 to #8 from August, 1970 to September, 1971, sharing space with the Inhumans, no less), before moving over to Daredevil (from Daredevil #81 to #124, during which, Natasha finally joined the Avengers in Avengers #111 in May, 1973 briefly, and then moved on to a new super-hero team, the Champions).