The aircraft inadvertently landed gear-up. Probable Cause (translated from Swedish): During approach for landing runway 29 Link”ping / SAAB’s airport, the aircraft’s landing gear has not been deployed. Shortly before the intended landing, the aircraft, which so far has been flown by the other pilot, had been at such a low height that its two propellers and underside collided with the runway. In connection herewith, propeller tips broke off and hit and penetrated into the aircraft body, whereby several rudder were damaged. After the commander took over, he managed to get the aircraft up in the air. Despite the lack of steering action, he was able to control the aircraft and carry out a belly landing on the grassy area west of the landing threshold by changing the throttle levers alternately on the right and left engine. During the belly landing, the aircraft collided with the runway threshold, whereby the aircraft suffered significant structural damage. Contributing factors have been – lack of cockpit discipline, – failure to comply with the prescribed landing procedure with regard to deployment of landing gear and checking that this has happened. – the low audibility of the landing sound warning.