Quite possibly the first all-metal airplane, the JL-6 (also called the F-13 in Europe) was designed by renowned German aircraft designer, Hugo Junkers. The JL-6 was powerful, spacious, and had an enclosed cockpit, in which two pilots could work the controls. This made long distance flying much easier.
Unfortunately, the JL-6 had problems with fuel distribution. The hoses that connected the fuel tank to the engine had a tendency to become disconnected when under stress. Thus, if the plane took a hard turn, hit bad turbulence, or shook badly, the engine would begin to leak large amounts of fuel. This would cause the engine compartment to flood with fuel, and when the engine became starved, due to the lack of fuel caused by the leak, it would backfire, sometimes causing the pool of fuel below it to catch fire, usually resulting in the death of the aircrafts occupants and the destruction of the plane. The Airmail mechanics found and fixed this problem, however, not before three of the six JL-6s that the Post Office Department bought were destroyed in this manner.