During the taxi to the runway, the flight crew noted a vibration in the airplane and stopped to check the tires, thinking that they were underinflated. No obvious problem was seen, and they elected to takeoff. Shortly after takeoff, a fire erupted in the left main gear wheelwell. The pilot returned to the airport and landed safely. However, the airplane sustained extensive damage to the left wing and aileron control system. Examination of the brakes revealed a cocked thermal insulator in the piston bore. There was evidence that the brakes were worn, had been dragging, then overheated; and that the tires had exploded prior to the fire. The fire damage indicated that the fire had started near the brakes. An aluminum fitting was used as part of the hydraulic brake system on the steel brake housing. The exam indicated that the fitting had been installed but could not be found after the fire. The crew said that they delayed lowering the gear, in contradiction to the checklist requirement when the wheelwell and wing overheat light is on.