On July 1, 2007, about 1024 Pacific daylight time, a Piper PA-24-250, N6370P, landed with the landing gear retracted near Boulder City, Nevada. Nevada Steel was operating the airplane under the provisions of 14 CFR Part 91. The private pilot and one passenger were not injured; the airplane sustained substantial damage. The cross-country flight departed Kingman, Arizona, about 0945 mountain standard time, with a planned destination of Boulder City. Visual meteorological conditions prevailed; no flight plan had been filed. The pilot stated that he had performed his before landing checklist while on the downwind for runway 9R. He continued the approach, did not perform a final check, and forgot to extend the landing gear. During the landing flare, the airplane's belly touched down on the runway surface. The airplane skidded approximately 150 feet along the runway surface before coming to rest in the vicinity of the intersection of runways 9R and 33. The airplane immediately caught fire, and the pilot and passenger exited through the passenger door. The pilot stated that he did not observe the position of the landing gear handle immediately after the accident, but that the airplane co-owner visited the wreckage later that day and observed the handle in the up (retracted) position. The pilot stated that the airplane and engine had no mechanical failures or malfunctions during the flight.