By 1953, the research team had produced the Firebird XP-21, later referred to as the Firebird I, which was essentially a jet airplane on wheels. It was the first gas turbine powered car tested in the United States. The design is entirely impractical, with a bubble topped canopy over a single seat cockpit, a bullet shaped fuselage made entirely of fiberglass, short wings, and a vertical tail fin. It has a Whirlfire Turbo Power gas turbine engine, which has two speeds, and expels jet exhaust at some . The entire weight of the car is .
At first, Conklin was the only person qualified to drive it, and he tested it up to , but upon shifting into second gear the tires lost traction under the extreme engine torque and he immediately slowed down for fear of crashing. The car was later test driven at the Indianapolis Speedway by race car driver Maury Rose. The car was never actually intended to test the power or speed potential of the gas turbine, but merely the practical feasibility of its use. The braking system differs from standard drum systems, in that the drums are on the outside of the wheels to facilitate fast cooling, and the wings actually have aircraft style flaps for slowing from high speed.
The second prototype, the Firebird II in 1956, was a more practical design: a four-seat, family car. It is a low and wide design with a large dual air intakes in the front, a high bubble canopy top, and a vertical tail fin. Its exterior bodywork was made entirely of titanium. The engine output was , and to solve the exhaust heat problem it was fed through a regenerative system, which allowed the entire engine to operate at nearly cooler, and also power the accessories. Another innovation on the car was the first use of four wheel disc brakes, with a fully independent suspension, as well as a sophisticated guidance system which was intended to be used with "the highway of the future", where an electrical wire would be embedded into a roadway to send signals that would help guide future cars and avoid accidents.
The third design, the Firebird III, was built in 1958 and first shown at Motorama in 1959. It is another extravagant prototype with titanium skin, and no less than seven short wings and tail fins that were tested extensively in a wind tunnel. It is a two-seater powered by a Whirlfire GT-305 gas turbine engine, and a two cylinder gasoline engine to run all the accessories. Its exterior design features a double bubble canopy, and more technical advancements to make it more practical, such as cruise control, anti-lock brakes, and air conditioning. It also featured "space-age" innovations, such as special air drag brakes, like those found on aircraft, which emerged from flat panels in the bodywork of the car to slow it from high speeds, an "ultra-sonic" key which signaled the doors to open, and an automated guidance system to avoid accidents and "no hold" steering. The steering was controlled by a joystick positioned between the two seats. This gave the car a more futuristic feel and simulated the experience of flying a plane.