In 1947, the US Navy awarded the Ryan Company a contract to see whether or not a fighter plane could be made to take-off vertically. The result, after the Air Force took over the project in 1943, was the X-13. The aim at the beginning was to evaluate whether or not submarine-based aircraft would be feasible, and it's easy to imagine this beauty leaping from the seas. Once the Air Force became involved, the aim was to develop a jet-powered VTOL (Vertical Take-Off and Landing) aircraft.
The X-13, powered by a single Rolls-Royce Avon turbojet engine, was something of a success. During a test in 1957, it launched vertically, then transitioned to horizontal flight and back again. It was then demonstrated in Washington, D.C., where it crossed the Potomac River and landed at the Pentagon. Unfortunately, the Air Force chose not to develop the project any further, citing a lack of operational requirements. That said, today's Harrier and F-35 jets can trace their ancestry directly back to the X-13.
Reconnaissance is important in the field. In the 1950s, the US Army wanted a simple personal helicopter that could be operated by pilots with limited flight experience and with a small amount of instruction. The HZ-1 was seen as a potential motorcycle of the air, and early tests showed quite a lot of promise. Pilots could steer the craft simply by leaning in the desired direction, like an airborne Segway. Unfortunately, further study showed that the HZ-1 was too difficult for untrained hands to control. The project was cancelled.
F2Y Sea Dart
Only five prototype Convair F2Y Sea Darts were ever made, the result of a 1948 U.S. Navy competition aimed at developing a supersonic interceptor aircraft. Although the F2Y's life was relatively short, it holds one record -- it's the only seaplane to ever travel faster than the speed of sound.
In November of 1954, the Sea Dart disintegrated in mid-air during a demonstration for the Navy and the media, killing its test pilot. That was the end of program, but the Navy had been losing interest for a while - problems with supersonic fighters on aircraft carriers had been solved, and airplanes like the Sea Dart were no longer necessary.
You can be forgiven for thinking that the above flying wing heavy bomber was developed in the 1980s or 1990s. In fact, the YB-49 was designed and constructed just after World War II. It was passed over for a much more conventional Convair design, the B-36.
The first prototype suffered massive engine failure and the second came down in 1948, killing its pilot (Captain Glen Edwards, after whom Edwards Air Force Base is named). The aircraft suffered structural failure and the outer wing sections became detached from the center section, effectively putting an end to the program. Still, the project has a neat coda: In 1980, Jack Northrop, Northrop's elderly and wheelchair-bound founder, was taken back to his firm's headquarters. Once there, he was taken to a top-secret area and shown a model of the Air Force's plans for the then-new Advanced Technology Bomber, a.k.a. the B-2A. It was a flying wing. Northrop - who was long mocked for championing the flying-wing idea - is said to have exclaimed "I [now] know why God has kept me alive for the past 25 years."
When it was developed in 1950, the XC-120 was unique. Its huge removable cargo pod was positioned below its fuselage and intended to speed loading of cargo -- the pod could be removed, a new one installed, and the plane made ready for take-off in relatively short order.
Three pod designs were proposed: one for troop transport and paratrooper deployment, one for cargo only, and one that was open to the elements and could be jettisoned in mid-air, landing with the aid of a parachute. The XC-120 was tested extensively and made appearances at a number of air shows in the 1950s, but it was eventually scrapped in favor of more traditional cargo-carrying aircraft.
The Goblin, nicknamed the "flying egg," was conceived during World War II and intended to be a plane within a plane. It was intended to be carried within the bomb bay of the enormous Convair B-36. It was supposed to act as a defender -- a parasite fighter -- that would be dropped from the bomb bay of the mother ship in times of need, one that could harry enemy fighters while the B-36 went on its way. When its mission was complete and the enemy fighters dispersed, the Goblin was to hook into a trapeze system underneath the B-36 and return to its bomb-bay hangar. Because of this midair "landing" procedure, the XF-85 was never equipped with any kind of landing gear.
All in all a really cool idea, but the project was soon scrapped. The reason for its cancellation is almost painfully obvious: the Air Force decided that aerial refuelling was a much safer way to extend the range of its fighters.
Republic XF-103 Thunderwarrior
The Thunderwarrior was developed at the beginning of the Cold War as a high-speed bomber interceptor, but it never passed the mock-up stage, and work on the prototype was continuously delayed by engine problems. The nose of the aircraft was completely consumed by an enormous radar set that offered (for the time) very long ranges of detection. Its missiles were carried in bays on the side of the fuselage. The project was canceled in 1957.
A-12 Avenger II
The Avenger II still manages to look futuristic. It was part of a joint program between McDonnell Douglas and General Dynamics. It was intended to be based on aircraft carriers and act as an all-weather stealth bomber. It was planned to replace the aging A-6 Intruder.
The flying wing concept was back in vogue by the 1990s; the Avenger was in the shape of an isosceles triangle with the cockpit at its apex. The internal weapons bay could carry smart bombs and other air-to-ground ordinance. Unsurprisingly, people involved with the A-12 nicknamed it the "flying dorito." The project was canceled in 1991 on grounds of cost.
Convair XFY Pogo
The Pogo was an experiment in vertical takeoff and landing known as a tailsitter. It launched and landed on its tail, and was intended to be operated from small warships. Take-off was easy, but the problems really came with the landing: The pilot had to look over his shoulder to judge the distance between the plane and the ground while at the same time working the throttle to ease the plane down to its landing position.
Technical problems like those aside, had the project persisted, it would have meant that only the most experienced pilots could have flown the Pogo - putting one on every small warship would not have been feasible. However, with only half the speed of contemporary jet engine fighters (Mach 2 at the time), the project was put on hiatus in 1954.
We will finish with one that was successful -- well, almost. The Lockheed YF-12 was a prototype interceptor that spawned the SR-71 Blackbird. However, despite breaking numerous records during testing (including a speed record of 2,070.101 mph and an altitude record of 80,257.86 ft, both on May 1, 1965), the program ended in 1968. One word says it all: Vietnam. At the time, defense of continental America was less of a priority, and the project was shelved.