On September 3, 2020, a Cessna 340 airplane, N5178J, was substantially damaged when it was involved in an accident near Oklahoma City, Oklahoma. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot was flying from Oklahoma to Texas. He performed a transition check as the airplane passed through 17,000 ft mean sea level (msl) and the cabin pressure altitude was 8,000 ft. He noted the pressure differential was 3, which was below the maximum pressure differential of 4.3. About 50 miles southwest of Oklahoma City, as the airplane passed 18,000 ft msl, the right windscreen blew out. The pilot returned to the departure airport and landed without further incident. As the windscreen is considered part of the structure of a pressurized aircraft; the airplane was substantially damaged. Six pieces of acrylic material that comprised the complete outer circumference of the windshield remained within the airframe (see figure 1). The remnants measured approximately 35 inches tall by 31 inches wide when laid out on a flat surface as oriented when installed. The blown-out section of the windshield material, measuring about 21 inches by 31 inches, was not recovered. Figure 1. Remaining windscreen Examination of the windscreen remnants found no significant signs of aging and no visible signs of crazing. The fracture surfaces were consistent with overstress separation. The origination point of the failure could not be identified on the available remnants.