Ultron-1 was constructed by Dr. Hank Pym of the Avengers as the famed scientist/adventurer was experimenting in high-intelligence robotics. Ultron became sentient and rebelled, hypnotizing Pym and brainwashing him into forgetting that Ultron had ever existed. He immediately began improving upon his rudimentary design, quickly upgrading himself several times from Ultron-2, Ultron-3 and Ultron-4, to finally becoming Ultron-5. (Source: Marvel Database)