History
Bucky was stuck under an overturned Jeep alongside 4 of his fellow soldiers. If Iron Man hadn't swooped in and stopped a tank that was headed right for them, Bucky would've lost more than the temporary use of his legs.[1]
Barnes latered volunteer to be the second Iron Man at Reed Richards' behest, since he felt he owed a debt to Steve Rogers for saving the lives of everyone in his platoon. He never got the chance, as Galactus invaded and Tony Stark wore the armor instead in the final showdown.[2]