The only skin on a 17 that is .020 is the nose back to frame three right forward of the cockpit.They were not designed to survive 25 missions but were expected to last 50 hours.Remember Boeing built it so it was built tough.None of the jets will be flying as long as a B-17.The oldest is 65 years and flying and the youngest is 63 years iold. I was thinking jets used 7075 T7351 because of the resistance to corrosion that T-7 offered.