This one has been confirmed by Marvel fairly early on. In the comics, Ultron is the creation of Hank Pym, a mercurial member of the Avengers who has gone by the code names Giant Man, Goliath, Yellowjacket, and even stole the name of the wife he infamously abused The Wasp for a brief, ill-advised period. But you probably know him best as the original Ant-Man. Pym will be introduced to the Marvel Cinematic Universe after Age Of Ultron, as the mentor to Scott Lang's Ant-Man, played by Paul Rudd. Which means that Ultron's origin is going to be retooled not as a creation of Pym that turns on the Avengers, but of Tony Stark. That still turns on the Avengers. In Age Of Ultron he's going to be created as part of a programme to try and better police the world than a handful of superheroes can on their own. Except he programmes the AI a little too well, the robot becomes self-aware, and turns on his creator. Much like in the comics, made for benevolent purposes that turn malevolent just at someone's else's hand/expense.