Two heads are generally better than one, as they usually say. With this theory in mind, one would think it could be applied to plane design. And thus the North American F-82 Twin Mustang was born.
Unfortunately, things did not go according to plan and the “double P-51” that was designed as a long-range escort fighter for WWII, was called off before it was even operational.
Mil V-12
Russians traditionally have quite the knack for engineering, especially when it comes to helicopters, but when it came to this one, in an ironic twist, it was an issue of competing technologies that drove this helicopter into redundancy. In fact, you might say that they just "out-technology-ed" themselves. Considered to be the world's largest helicopter, the wondrous flying machine could carry up to 196 passengers, or 88,000 pounds of payload.
But its downfall was that it was unable to meet missile advancements. Although huge, its purpose was deploying ballistic missiles. Missile technology moved at lightning speed and during the '70s, and the aircraft became outdated as missiles moved forward without its help.
Bartini Beriev VVA-14
If this thing had a spirit animal, it would be a walrus. To be honest, at the first glance at this monstrosity, one struggles to figure out what exactly it is. Is it a boat with wings? A plane? No one is entirely certain, but one thing is clear, it looks very bizarre.
Designers from the Soviet Union developed this machine in the 1970s as a wing-in-ground-effect aircraft to take off from water and fly at high speed over extended distances. The designers meant for the plane to fly at high altitudes, as well as hovering just above the sea utilizing aerodynamic ground effect. The first flight was in September 1972. There were not that many since and now it's just an old weird plane.
Dornier Aerodyne
Engineer Alexander Lippisch was in charge of the Dornier Aerodyne, a wingless VTOL bot-controlled winged aircraft. The thought that Lippisch had conceptualized was for the plane was to merge lift and thrust production into a single unit and flow channel. Lippisch designed it to be a land or ship-supported drone for aerial reconnaissance.
Measurements-wise, the Aerodyne was 18 feet long and 6 feet wide. Lippisch ended up being part of the Dornier team that produced only one prototype. For some mysterious reason, the first flight was in September 1972 and never again as it retired two months later.
EL/M-2075 Phalcon
According to FAS, the EL/M was the most advanced airborne early warning and control system. Had it not been for the clown-style nose of the EL/M, it would just appear to be a regular 707 Boeing. The chief objective of the plane was to serve as an intelligence provider to maintain air control and conduct surveillance.
The EL/M-2075 is a solid-state conformal array radar system for use on a 707 Boeing. While it was super-advanced, design-wise, people had an issue with the strange-looking nose making the Phalcon looking very strange. You can't win it all.