After the blistering pace of aircraft development during the 1940s and 1950s, it was inevitable that things would slow down.  And they did.  By the 1960s, designers had a pretty good idea of what worked (swept-back wings) and what didn't (tail-sitters), the American and Soviet governments concentrated on proven concepts, dramatically limiting the amount of money and energy being poured into experimental aircraft.

As a result of this consolidation, the number of X-Plane and prototype models that came out of the Kennedy and Johnson years was limited.  Besides, by this time the public's imagination was no longer focused on the skies -- but on the stars.

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X-Planes & Prototypes

1900-1930

1940s

1950s

1960s

1970s

1980s

1990s

2000s

2010s