As time went by, the speed of airplanes began to increase. From the famous 12 mph top-speed of the Wright Brothers Kitty Hawk Flyer to the 400+ mph of the famous Army Air Force (later USAF) fighter P-51 Mustang, used in
World War II.
Even though the velocity of newer aircraft was increasing, very soon it was realized that an invisible "barrier" was preventing aircraft from surpassing the speed of sound. This barrier was known as the famous "sound barrier". The barrier took the life of several test pilots as they attempted to break the "sound barrier" in the famous experimental aircraft, such as Bell X-1. In 1947, a young test pilot named Chuck Yeager broke that barrier and exceeded the speed of sound. From that point on a series of experimental supersonic aircraft took to the sky breaking speed record after speed record.
Today we still can see some of the supersonic aircraft that were built in the 1960's; for example - The British Concord (cruise speed: Mach 2.0), the Russian TU-144 (cruise speed: Mach 2.2), and the famous American spy plane SR-71 Blackbird (cruise speed: classified but known to be over Mach 3.0).