I was just thinking about all of the proposed planes that never made it beyond clay mock-ups and the prototype stage. Some of these planes may have been successful, others were ill-conceived. What I want y'all to do is post info about planes that never were for one reason or another. Here is my contribution:
In the mid-1960's Piper Aircraft Company was looking to build a new mid-sized light twin based on the Cherokee Six, sized between the Twin Comanche and the Aztec. This plane would later become the Seneca, but before it came out Piper did something rather odd. The original plane was a tri-motor. The first prototype flew with the Cherokee Six's 250hp Lycoming O-540 in the nose with two 115hp Lycoming O-235s on the wings. This design was horribly flawed, with the center engine out the 235s could not keep the plane flying. Still bent on the tri-motor design Piper decided to use the 150hp Lycoming IO-320, which had been successfully used in the Twin Comanche. Again not enough power with an engine out and windmilling. In order to reduce costs Piper fitted the tri-motor with fixed pitch props, which furthered the problems of an engine out. In 1972 Piper finally released the Seneca I, this time powered by two Lycoming IO-360s producing 200hp, and later the Seneca II and on were fitted with two Continental TSIO-360s. Nobody really knows why Piper toyed with the tri-motor for so long... the last tri-motors that were in people's minds was the Ford Trimotor and the Junkers Ju-52, the extruded aluminum machines probably would not have left a desirable past on a Piper tri-motor.