The Iron Giant was a remarkable film. In that film, the Iron Giant is a remarkable character.
Was he a manufactured weapon? Or just a member of a species of living robots, and suffering from PTSD? What was the cause of the war he fought in? How did he end up on Earth? Are other of his kind on their way? Is he AWOL?
It seems that there is no canonical back story for the Iron Giant, besides that he fought in a war and was very well armed.
The Iron Giant is based on a far less down-to-earth, more poetic book called The Iron Man. In that, the lack of origin and unknowability is very much part of the point of the story.
So no, there are no stories that explain his backstory, beyond the standard "Terminator" plot that is hinted at in the film.
The original Iron Man is certainly not unique, as evidenced by the arrival of a second Iron Woman in the sequel novel by Ted Hughes. Hogarth's assumption is that the giant comes from the Earth rather than from outer space.
Ever since the Iron Man had made Hogarth so famous, all kinds of people sent him letters. But this was the strangest yet. He sat on his bed rereading it, and looking at the snowdrop.
He'd often wondered if the Iron Man had any relatives, somewhere. They'd be hidden away, of course. Quite likely in some deep mudhole. Or in the sea. Or inside the earth. After all, the Iron Man had come from somewhere. Why shouldn't there be others?
Note that although the giant has the appearance of a robot, it's not actually robotic, more of a facsimile of a robot than a robot.
Lucy could now see her clearly in full daylight. She gazed at the giant tubes of the limbs, the millions of rivets, the funny concertinas at the joints. It was hard to believe what she was seeing.
"Are you a robot?" she cried.
Perhaps, she thought, somebody far off is controlling this creature, from a panel of dials. Perhaps she's a sort of human-shaped submarine. Perhaps . . .
But the rumbling voice came up out of the ground, through Lucy's legs:
"I am not a robot," it said. "I am the real thing."
And now the face was looking at her. The huge eyes, huge black pupils, seemed to enclose Lucy—like the gentle grasp of a warm hand. The whole body was like a robot, but the face was somehow different. It was like some colossal metal statue's face, made of parts that slid over each other as they moved. Now the lips opened again, and Lucy almost closed her eyes, she almost shivered, in the peculiar vibration of the voice:
