On December 4, 2017, about 0720 Pacific standard time, a Beechcraft A36, N251E, was substantially damaged when it was involved in an accident near Rio Oso, California. The pilot received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, he was flying from McClellan Airfield (MCC), Sacramento, California, to Yuba County Airport (MYV), Marysville, California. The preflight, engine start, taxi and takeoff were normal from MCC. While in cruise flight, the pilot noticed a “flicker” below his seat. When he checked under the seat, he observed that a fire had started. He then identified a field to make an emergency landing. The pilot stated that he landed gear up in a plowed field, where the airplane came to rest upright facing about 150° from its original direction of travel. After the pilot exited the airplane, it burst into flames; the airplane cockpit was thermally damaged. An examination of the airframe near the left side of the airplane revealed burnt electrical wires and a hole in a fuel line near the area of the fuel valve inside the cockpit. The fuel selector valve trim panel with attached wires, a fuel flow valve with fuel lines, and a portion of electrical wiring were submitted to the National Transportation Safety Board Materials Laboratory in Washington, DC, for further examination. Examination found that the trim panel for the fuel selector valve had melted and missing insulation, which were consistent with exposure to a fire environment. The attached wiring did not exhibit electrical arcing. The fuel selector valve had sooting present on some of the fuel lines and a rough spot and pin hole near a 90º bend were observed. The rough spot exhibited cratering and re-solidified metal splatter that adhered to the fuel line surface and the hole through the fuel line wall. Both features are consistent with electrical arc erosion of the fuel line. The trim panel and fuel selector valve were positioned as they would have been installed in the accident airplane. The damaged portion of the fuel line aligned with the damage on the trim panel. A portion of electrical wiring was examined, and the terminal connector exhibited electrical arc corrosion.