On April 5, 1997, about 1030 eastern standard time, a Beech 100, N402G, registered to and operated by ISO Aero Service, Inc., experienced failure of the pilot's "D" oversize window while climbing to cruise near Kenly, North Carolina. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 positioning flight. The airplane sustained minor damage and the airline transport-rated pilot, the sole occupant, was not injured. The flight originated about 15 minutes earlier from the Kinston Regional Jetport at Stallings Field, Kinston, North Carolina. The pilot stated that while climbing through 12,000 for 12,500 feet, at a cabin differential pressure of 4.3 psi, the pilot's flight compartment side "D" window separated. He diverted to his departure airport and landed uneventfully. At the time of the window separation the cabin altitude was about 2,000 feet. The remaining portion of the failed window was sent to the NTSB Materials Laboratory in Washington, D.C. Examination of the window revealed evidence of preexisting cracks that originated at the external surface of the window at the transition radius between the window flange and the center portion of the window. Review of the maintenance records for the airplane revealed that the failed window was installed on February 13, 1986, in accordance with an Airworthiness Directive. At the time of failure, the airplane had accumulated 4,008 hours since the window installation. Further review of the maintenance records revealed that it was last inspected on October 10, 1996. The airplane had accumulated about 240 hours since that inspection at the time of failure. The airframe manufacturer inspection requirements for the failed window does not require the removal of the metal retaining ring to examine the entire area of the window for cracks if none are visually observed on the outer surface of the window during the initial inspection. Additionally, a detailed inspection of the failed window is required by the manufacturer to be accomplished only every 600 hours.