EVERY new model of airplane starts life as an "Experimental" ... even those built by OEMs. The second classification adds "Amateur Built", "Factory Built" or etc. In other words, the Cessna Citation X prototype (N750CX) is an "experimental" airplane (and still is). What people refer to as being "certified" is really short for "Type Certified", which means that every single part, nut, bolt, tube, cable, wire, etc. is conformed (built to a drawing and its tolerances) and meets very stringent FAA regulations. Airplanes with a TC are truly "thoroughly tested" with typically 1000+ hours of flight time and several hundred stalls (and spins if 14CFR23).