The aircraft made an emergency off-airport wheels-up landing shortly after takeoff. The pilot said all of the pre-flight, before takeoff and takeoff checks were normal. Shortly after takeoff, the aircraft started to lose airspeed. All controls were checked as the nose was lowered to maintain airspeed. The pilot later said he did not check or does not remember what the fuel flow or manifold pressure were. No mention was made of RPM or carburetor heat at any time. Since the airspeed continued to fall, the pilot lowered partial flaps and landed straight ahead in an open field. A fence was penetrated during the ground slide of about 75 yards. The engine ran fine on a post-accident check. No malfunctions could be found. The pilot was questioned about before takeoff procedures and he said he set the mixture at 1700 RPM. The owner's manual says the mixture should be set, but does not say what power setting to use. According to a Beechcraft spokesman, the mixture should be set at full power. The pilot said all frost was removed before takeoff. Density altitude was 2911 ft.