27 May 1958: At Lambert Field, St. Louis, Missouri, McDonnell Aircraft Corporation’s Chief Test Pilot (and future company president) Robert C. Little made the first flight of the YF4H-1 prototype. The twin-engine Mach 2+ airplane was the first pre-production model of what would be developed into the legendary F-4 Phantom II fighter bomber.
The flight lasted 22 minutes. Little had planned to go supersonic but a leak in a pressurized hydraulic line caused him to leave the landing gear extended as a precaution, should the back-up hydraulic system also have a problem. This limited the maximum speed of the prototype to 370 knots (426 kilometers per hour). Post-flight inspection found foreign-object damage to the starboard engine.
Source: Thank you to Bryan R. Swopes creator of This Day in Aviation for this information