On May 18, 2023, at 0817 central daylight time, a Beech 35-C33A, N63B, was substantially damaged when it was involved in an accident at Indianola Municipal Airport (IDL), Indianola, Mississippi. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot stated that he had flown the airplane more than 6 hours in the week before the accident. On the day of the accident flight, an engine runup was completed with no anomalies noted. After departure, when the airplane was about 800 to 1,000 ft above ground level, the pilot smelled “burning plastic” and observed smoke from the engine compartment. He turned the airplane back toward IDL and was on the base leg of the traffic pattern when he observed flames coming through the firewall under the battery box area. He tried to kick out the flame, but the fire seemed to be coming from the engine side of the firewall. He turned off the electrical system and adjusted the mixture control to cut off the fuel. The pilot performed a gear-up landing on the runway as the cabin filled with smoke. After the airplane came to rest and the pilot egressed, the fire consumed a majority of the fuselage before it was extinguished. The pilot recorded a video of the airplane on the runway after he egressed. Figure 1 is a still image from the video that shows dark smoke in the cabin and heat discoloration below the front right windscreen, near the battery box. Figure 1. The airplane on the runway with dark smoke in the cabin and heat discoloration below the front right windscreen, near the battery box. The maintenance records revealed that an annual inspection was completed on August 2, 2022, during which the battery was removed, serviced, and reinstalled, and a defective fuel boost pump was replaced. On February 1, 2023, the throttle and mixture cables were replaced. According to the pilot, they were replaced because the old mixture cable was slipping. After the accident, the responding Federal Aviation Administration inspector examined the wreckage and stated it appeared that the fire had started below the battery box. He could not tell if it originated on the engine side or the cockpit side of the firewall. He removed the end cap from the starter and the internal components did not reveal any anomalies. All of the electrical wiring in the cabin area was burned down to bare copper wire. Also, he found no compromised fuel lines on the engine side of the firewall. He identified one melted fuel line on the cockpit side of the firewall, which was behind the pilot’s (left) side of the instrument panel. Portions of the fuel line had melted around an adjacent avionics wire, which exhibited damage to the outer shielding. Due to the significant fire damage, the inspector was unable to determine the source of the fire.