The pilot reported that after takeoff from runway 31, the airplane climbed to approximately 75 feet above ground level and encountered a downdraft. The airplane impacted the runway, separating the main landing gear. The right wing spar was bent and crushed aft, and the fuselage bulkhead at the left wing attach point was crushed. An examination of the airplane's systems revealed no anomalies. Prior to departure, the pilot estimated the wind as 280 degrees at 15 knots. After the accident, a hand held wind meter recorded the winds at 8 knots, gusting to 18 knots.