On June 14, 2003, at approximately 1300 mountain daylight time, a Beech S35, N5751K, was substantially damaged during a gear-up forced landing north of the City of Colorado Springs Municipal Airport (COS), Colorado Springs, Colorado. The private pilot and his two passengers were not injured. Visual meteorological conditions prevailed. An instrument flight rules flight plan had been filed for the cross-country flight being conducted under the provisions Title 14 CFR Part 91. The flight originated from Fond Du Lac, Wisconsin, at approximately 0830. According to the pilot, he filled the fuel tanks to capacity before departure and calculated that he had approximately 7 hours of fuel on board. Flight time was estimated to be 5 hours and 10 minutes. After takeoff, the pilot was cleared direct to Colorado Springs, Colorado. The flight went as planned until arrival in Colorado. Denver Air Route Traffic Control Center (ARTCC) gave the pilot several vectors, due to weather, that he had not planned on. The pilot stated that he lost track of his fuel situation and failed to switch tanks during this time. While on approach into Colorado Springs the engine lost power. The pilot elected to touch down with the landing gear up and the flaps retracted. During the forced landing, the airplane's keel beam and engine firewall were bent, and the fuselage was wrinkled forward of the right wing. An examination of the airplane's systems revealed no anomalies. According to the pilot, when a salvage company retrieved the airplane from the field, approximately 2.5 hours of fuel was found in the airplane.