Specifications

The X-2 was the first plane to break Mach 3.  Unfortunately it went into a spin which resulted in the loss of pilot and aircraft.

Aircraft - X-2 (U.S.A.)

The X-2 was designed to go over 100,000 ft. and fly above Mach 3.  It was the first plane to go over Mach 3, but on that flight Milburn "Mel" Apt lost control of the plane and he and the plane were lost.  The plane was designed to test the problems associated with high supersonic travel such as the severe heating effects caused by aerodynamic friction.

"While the X-2 had delivered valuable research data on high-speed aerodynamic heat build-up and extreme high-altitude flight conditions, this tragic event terminated the program before the National Advisory Committee for Aeronautics could commence detailed flight research with the airplane and the search for answers to many of the riddles of high-Mach flight had to be postponed until the arrival, three years later, of the most ambitious of all the rocket planes -- the X-15."
Source: GlobalSecuirty.org

Specifications of the X-2

First Flight: November 18, 1955
Mission: Determine affects of supersonic and subsonic speed on winged aircraft
Major Accomplishments: First aircraft to exceed Mach 3, on flight which resulted in fatal accident with pilot Captain Milburn Apt.

Power Source: One (1) Curtiss-Wright XLR25-CW-1 rocket motor. 15,000 lb  thrust (6,804 kb).
Wing Span: 32 ft. (9.75 m)
Length: 44 ft. (13.41 m)
Maximum Achieved Speed: Mach 3.196 (2,094 mph)