The Bell X-1 broke the sound barrier. Although highly advanced , the United States was still looking to develop another test bed that would go even faster and solve problems with flight at higher speeds. The first problem that needed to be solved was the thermal barrier. Extreme heat resulted from the friction in transonic speeds. So the X-2 was a test bed for materials scientists as well. Stainless steel was mostly used but other composite materials and alloys were created just for this plane. Normal hydraulic control systems and control surfaces also had to be upgraded to keep the plane in steady flight.
The two X-2s, both manufactured at Bell’s factory in Niagra Falls in 1955, were destroyed in tests – but not before both set absolute world speed records of Mach 2.87 and over Mach 3.The first one was dropped from the bomb bay of a B-50 bomber, basically a B-29 upgraded for higher altitudes. It crashed in Lake Ontario. The second X-2 crashed in September 1956 and Pilot Milburn Apt was killed in the accident.
No prototypes remain.
Sources: X-Planes, Wikipedia