XC-99 Hospital Ship - This drawing shows the proposed flying hospital version of the Convair XC-99, which would have carried 305 litters. (Click on image above for a larger version.) The XC-99 was a prototype heavy cargo aircraft built by Convair for the US Air Force. It was the largest piston-engined land-based transport aircraft ever built.
Howard Hughes's H-4 Hercules - The "Spruce Goose" - It flew only once on Nov. 2, 1947. The Hercules is the largest flying boat ever built and has the largest wingspan of any aircraft in history. They really weren't sure it would lift off - but it did.