According to the pilot of the retractable gear-equipped airplane, while the airplane was on the downwind leg of the traffic pattern, he lowered the landing gear handle and thought he saw the green landing gear indicator lights illuminate. He then became distracted and did not check the landing gear lights again. The airplane touched down with the landing gear retracted, resulting in substantial damage to the fuselage. The pilot stated that the accident was a mistake and that his own fatigue or stress may have been an issue. Examination of the airplane by a Federal Aviation Administration inspector revealed that the landing gear handle was in the "down" position, the landing gear indicator lights were functioning, and that the landing gear motor circuit breaker had popped. The circuit breaker was reset and the landing gear was cycled six times, with no mechanical malfunctions or anomalies noted.