Phil, all due respect, but your reply is just shoulda-woulda-coulda and wishful thinking. The problem was NOT availability of materials The problem was finding an engine of sufficient power for the size of the aircraft. The HK-1 could not have flown before a sufficiently powerful engine was available.
I must reiterate that the engines Hughes eventually used (Pratt &Whitney Wasp Major) was not developed for production until 1944, and not available in any numbers until 1945. Advances in engine technology and supercharging during WW2 proceeded very quickly, yes. But seriously, there was no engine in the production that could have made the HK-1 fly in time for D-Day in 1944.
Again, the most critical component the HK-1 needed was an engine of sufficient power. Let's look at the 3 aircraft you mentioned (plus the workhorse DC-3) and it becomes readily apparent why:
Aircraft Empty Weight (lb) Gross Weight (fully loaded and fueled) Engine HP Total HP HP/Wt Ratio
Pan-Am Clipper (Boeing 314): 48,400 84,000 Wright Twin Cyclone (4) 1600 hp 6400 0.74
Boeing B-29 74,500 120,000 Wright Duplex Cyclone (4) 2200 8800 0.73
Hughes HK-1 250,000 400,000 P&W Wasp Major (8) 3000 hp 24,000 0.60
Douglas DC-3 16,800 25,200 P&W 1800 Twin Wasp (2) 1200 2400 0.95
Even running 8 Wasp Majors, the HK-1's horsepower to weight ratio was really low. Hughes said after the single flight of the aircraft that he would not fly it again until "further development" could be done. The only thing that could realistically help it was to put more power into it, and that would have required (probably) jet engines.
There is no way the Wright R-3350 could have powered the HK-1. Eight R-3350s would have made only 17,600 hp, for a horsepower to weight ratio of 0.44.
We might wonder if they had constructed it out of aluminum instead of wood, it might have been a bit lighter, but that's comparing apples to oranges... the construction techniques for wood vs metal are completely different. There is only one example I know of that has some relevance, and that is comparison between the DeHavilland Mosquito (wood) and the Argentine I.Ae.24 Calquin (aluminum). The Argentines basically duplicated the shape and size of the Mosquito in metal. The Calquin was about 18% lighter than the mosquito when empty, but when fully fueled and loaded was identical in weight. So even if they had made the HK-1 of aluminum, they would still have needed to wait for the Wasp Major to get it into the air.
