Republic XF91 Thunderceptor Rocket Fighter

When post-war budget cuts ended many combat aircraft programs, the overriding importance of one of the most promising of these, known as Secret Project Number MX-809 (MX meaning Material, Experimental) kept it alive.

The MX-809 project was created in hope of fielding a very high speed, very high altitude daytime interceptor pursuit airplane capable of "meeting and defeating any strategic bomber aircraft that any potential adversary might produce." In other words, it was to be used for the sole purpose of defending U.S. airspace.