The World War II German design, the Arado Ar 232 was the first purpose built cargo aircraft. The Ar 232 was intended to supplant the earlier Junkers Ju 52 freighter conversions, but only a few were built. Most other forces used freighter versions of airliners in the cargo role as well, most notably the C-47 Skytrain version of the Douglas DC-3, which served with practically every Allied nation. One important innovation for future cargo aircraft design was introduced in 1939, with the fifth and sixth prototypes of the Junkers Ju 90 four-engined military transport aircraft, with the earliest known example of a rear loading ramp. This aircraft, like most of its era, used tail-dragger landing gear which caused the aircraft to have a decided rearward tilt when landed. These aircraft introduced the "Trapoklappe", a powerful ramp/hydraulic lift with a personnel stairway centered between the vehicle trackway ramps, that raised the rear of the aircraft into the air and allowed easy loading. A similar rear loading ramp even appeared in a somewhat different form on the nosewheel gear-equipped, late WW II era American Budd RB-1 Conestoga twin-engined cargo aircraft.