A lot of design studies by Dr. Mader were developed, when the F13
design work was finished. In August 1919 a single-seated sport aircraft was purposed, as well as three-seated
passenger aircraft. Both designs showed the highlever monoplane design, which was first used for the
Junkers Parasol during WW I. While the sport aircraft design got low priority,
the three-seated cabin aircraft was further investigated. At first Junkers and Mader decided to design this
small passenger aircraft as a cantilever version, but Philipp von Doepp
proved the better aerodynamics of a highlever monoplane in windchannel tests, so the design was changed.
To get more experience with the highlever monoplane design, an experimental aircraft was built under
the designator J15. On September, 4th 1920 the prototype (c/n 525) was ready for its first flight. Due to Allied
investigations at Junkers Flugzeugwerke, the aircraft was moved to the Netherlands and it seems, as if the
first flight was not performed before January, 25th 1921 by Zimmermann.
The J15 proved von Doepp's idea of a highlever monoplane design and the final decision was made to built
the three-seated passenger airliner in that way under the designator K16.