On August 1, 2005, about 2002 mountain standard time, the pilot flying a Beech 95-B55, N5888J, inadvertently landed with the landing gear partially extended at the Tucson International Airport, Tucson, Arizona. As the airplane decelerated it veered off runway 21, impacted a taxiway light, and was substantially damaged. Visual meteorological conditions prevailed during the business flight, and no flight plan had been filed. The pilot was not injured. The flight was performed under the provisions of 14 CFR Part 91, and it originated from Tucson about 1945. The pilot reported to the National Transportation Safety Board investigator that after takeoff he realized that he had forgotten documents for his business flight, so he returned to the airport and landed. No evidence of any mechanical malfunction was noted during the approach or initial portion of the landing rollout. As the airplane decelerated to about 60 knots and was rolling straight down the runway, the left main landing gear and the nose gear retracted. The airplane swerved left and directional control was lost. The pilot reported that he had used the pre-landing check list and lowered the landing gear. The airplane was subsequently examined under the direction of the Safety Board investigator. The examination did not reveal any mechanical anomalies with the landing gear retraction or extension system. The landing gear position lights operated normally. On the pilot's completed "Aircraft Accident Report" he indicated that his last aviation medical certificate was issued on June 7, 2005. According to Federal Aviation Administration (FAA) medical records, his last medical certificate was issued on June 7, 2001. In a letter to the FAA, the medical doctor whose name appears on the certificate indicated that he had reviewed a faxed copy of the medical certificate and that, except for the "5" in the year 2005, the medical certificate appeared exactly the same as the certificate the doctor had issued on June 7, 2001. The doctor further stated in his letter that he had only seen the pilot one time, and that was on June 7, 2001.