Heinkel He 116

The Heinkel He 116 was an extremely long-range mail plane designed to deliver airmail between Germany and Japan. Several examples were built for this role, as well as a small batch to be used in the long-range reconnaissance role.
Development and design.
The design started in response to a 1936 request by Lufthansa, which planned a route over the Pamir Mountains in Afghanistan. This was the primary difficulty in producing an aircraft able to meet the range requirements, because the aircraft would have to lift its large fuel load to to clear the mountains. At the time there were simply no engines available with that sort of altitude performance, although Hirth was working on one in the class. The Günter brothers proposed to adapt their basic He 70 "Blitz" airframe to carry four of these engines to provide enough power for the massive fuel load.
The He 116 would use a modified version of the He 70's two-spar elliptical planform, skinned (like the He 70) with plywood. The fuselage was all-new, consisting of a semi-monocoque duralumin body with watertight bulkheads in case the aircraft were forced down over water. The design shared enough construction with the He 70 that the first prototype He 116 V1 was completed in early 1937. The new engines were not ready at this point, so the prototype was instead fitted with the much smaller Hirth HM 508C of . 