On December 22, 2003, at 1649 central standard time, a Beech 95-B55, N221Q, piloted by a certified flight instructor (CFI), sustained substantial damage while landing wheels-up on runway 31 (4,115 feet by 150 feet, dry asphalt) at the St. Paul Downtown Airport (STP), St. Paul, Minnesota. Visual meteorological conditions prevailed at the time of the accident. The CFI and his dual student reported no injuries. The instructional flight was operating under the provisions of 14 CFR Part 91 without a flight plan. The local flight departed South St. Paul Municipal Airport (SGS) at 1513. According to the CFI's written statement, during the flight they had successfully completed several landings at STP prior to returning to SGS. The CFI reported that he and his student heard a "loud noise" during the last landing gear retraction. The CFI stated the landing gear did not fully extend and the gear-down lighting did not illuminate while on downwind for runway 34 at SGS. The CFI reported they attempted to retract the landing gear, but the gear-up light did not illuminate. The CFI stated they departed the traffic pattern and began to troubleshoot the problem. The CFI reported they attempted to extend the landing gear again; however, the cabin began to fill with smoke, and the gear motor and right generator circuit breakers both tripped. The CFI stated they performed the manual gear extension procedure without success. The CFI reported that "The manual gear extension handle was not catching anything and turning very loosely." The CFI stated they performed a tower fly-by at STP so that the tower personnel could assess the landing gear position. STP tower personnel reported that the gear doors were open but the landing gear was not extended. The CFI reported they landed with the landing gear retracted and subsequently impacted a runway sign. An inspector with the Federal Aviation Administration (FAA) performed the on-scene investigation. The landing gear extension/retraction system was examined while the airplane was elevated on jackstands. The inspector was unable to extend the landing gear using the electric or the manual gear extension procedure. The landing gear extension/retraction gearbox was disassembled for further inspection. The inspector reported that one of the actuator worm gear bearings was found in several pieces, resulting in the worm gear being lodged between the sector gear and the gearbox housing. According to the Beech 95-B55 maintenance manual, the landing gear extension/retraction gearbox is to be replaced or overhauled every 2,000 hours. The accident airplane's gearbox assembly had accumulated 2,137 hours since the last overhaul and 6,342 hours since new.