On July 25, 2004, about 2142 Pacific daylight time, a Piper Aerostar 602P, N663W, sustained substantial damage during a wheels-up landing on runway 31L at the Boeing-King County International Airport (BFI), Seattle, Washington. The airplane is owned by the pilot, and was being operated as a cross-country flight under the provisions of Title 14 CFR Part 91 when the accident occurred. Visual meteorological conditions prevailed at the time of the accident. The commercial pilot and passenger were not injured. The flight originated at Sandpoint, Idaho, approximately 1 hour and 20 minutes prior to the accident. In a written statement, the pilot reported, in part, "...Seattle approach assigned the visual approach into Boeing Field runway 31L and sequenced me between a Lear and a Citation requesting that I keep hold speed at 160 knots until DUANE intersection." During the approach the pilot was handed off to Boeing tower and was cleared for the visual approach. He reported "As I approached I slowed to gear extension speed and thought I had extended my gear (I had actually actuated my flaps)." Shortly thereafter, the airplane landed on runway 31L with the landing gear retracted. The pilot reported that there were no mechanical malfunctions or failures with the airplane that may have contributed to the accident.