The P-51 was not in fact, covered with steel. Most US planes, however, had a superior level of armor protection for the pilot. This might be what Soviet Ace is referring to. The IL-2 series was covered in steel, the entire fuselage, in varying thicknesses from 4-13mm, over a 5mm layer of duraluminum. The last is from the book "Complete Book of World War Two Aircraft" by Enzo Angelucci and Paolo Matricardi, 2001 updated edition, White Star Publishers. But then again, it's not a fighter.