Iron Man MK7: Tony Stark built his first suit of armor in a cave, out of salvaged missile parts. He has since upgraded his armor numerous times, using his scientific genius and personal fortune to create the most advanced piece of technology on the planet. The MK7 model is the one he uses as a member of the Avengers.
The Hulk: When a scientific experiment went wrong, scientist Bruce Banner was dosed with gamma radiation, triggering an uncontrollable transformation whenever he became stressed or angry. After years on the run from the U.S. military, Banner was tracked down and recruited to the Avengers Initiative.