On May 1, 2013, about 0745 central daylight, a Cessna P210N airplane, N731NR, was substantially damaged during a gear up landing at Spirit of St. Louis Airport (SUS), St. Louis, Missouri. The pilot did not report any injuries. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual meteorological conditions prevailed for the flight, which operated without a flight plan. The flight originated from St. Charles County Smartt Airport (SET), St. Charles, Missouri about 0700. While on left base of the traffic pattern at SUS, the pilot was instructed by air traffic control to make a short approach for traffic avoidance. The pilot stated that he lowered the landing gear handle, but was unsure if the landing gear down indicator light had illuminated. He normally checked the landing gear visually, but did not remember doing so on the approach of the accident. He did not observe an audible landing gear warning horn during the approach. The pilot stated that he was distracted with testing his air conditioner and thought he noticed an odd smell in the cockpit, which he thought may have been the air conditioner malfunctioning. The airplane touched down on the runway surface with the landing gear retracted, resulting in substantial damage to the fuselage structure and engine. During examination by maintenance and Federal Aviation Administration personnel, the landing gear was sequenced several times using both normal and emergency modes, with no abnormalities noted. The red gear up warning light, green gear down light and audible warning horn all functioned properly. The pilot stated that during the airplane's last annual inspection the audible warning system was noted to be out of adjustment. This issue was not duplicated during the examination. Examination of the landing gear system revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.