The pilot reported he completed the landing checklist on the downwind leg and confirmed that the landing gear was extended via the cockpit landing gear indicator lights. Additionally, he saw that the landing gear locking pins were engaged. On final approach, the pilot intended to move the flap control lever to the “full down” position; however, he inadvertently moved the landing gear control lever from the “down” position. The pilot did not perform a visual gear down check on short final. The airplane touched down with its main landing gear retracted and came to a stop on the runway. The pilot and his passenger exited the airplane and a post-accident fire ensued. The fuselage and left wing were substantially damaged by the fire. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.