The pilot of the multiengine airplane reported that the accident flight was the third flight since the most recent annual inspection, during which avionics upgrades were installed. Shortly after takeoff at night, the pilot experienced intermittent avionics malfunctions and elected to return to the departure airport. The pilot wanted to land as soon as possible and performed a straight-in approach with an approach speed that was 5 knots above what he normally flew. On short final approach, the pilot extended the flaps to the approach setting and thought he had extended the landing gear, but subsequently landed with the landing gear retracted. The airplane’s left wing and lower fuselage were substantially damaged during the landing. The pilot reported that there were no preimpact mechanical malfunctions or failures of the landing gear system.