On September 2, 1997, about 1020 central daylight time, a Learjet 31, N71JC, landed gear up at Aberdeen, Mississippi. The airplane was operated by North American Plastics, Inc. under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed. A visual flight rules flight plan was filed for the corporate flight. There were no injuries to the two airline transport pilots, and the airplane was substantially damaged. Origination of the flight was Madison, Mississippi, about 0955, on the same day. According to the pilot, the airplane was high and fast on final approach, because of restricted visibility in haze, and he executed a go-around. The pilot and copilot do not recall retracting the landing gear. According to the pilot, the flightcrew always used a checklist when flying. Generally, the pilot would do the items on the checklist with the copilot following along. During the second approach, the pilot stated he did not extended the gear because he was "sure in his mind that the gear was already down". The flight crew failed to extend the landing gear, resulting in the gear up landing. The airplane slid approximately 3,000 feet. Following the landing, the airplane caught fire under the right wing root, and the fire could not be extinguished with hand held fire extinguishers. Both pilots safely evacuated the airplane.