The pilot stated that while on an instrument landing system (ILS) approach for the medical life-flight, he was struggling to keep the airplane on the localizer and glide path. As the airplane descended through 800 to 900 feet above the ground, he broke out of the cloud layer but had difficulty locating the runway in the dark night conditions. During the landing flare he reduced the throttle and as the airplane touched down on the runway surface, he realized the landing gear were retracted; he did not hear any horns. The pilot recalled that when he selected the landing gear, he may have put the handle in neutral rather than keeping his hand on the lever and waiting for the three green lights to illuminate confirming the extended position. He did not follow his normal checklists because of the demands of the ILS approach. He also cited a few stressors in his life that may have contributed to a distraction including a change of airplanes and few ILS approaches performed in the recent six months. The pilot further added that there were no pre impact mechanical malfunctions or failures that would have precluded normal operation. The airplane incurred substantial damage to the lower fuselage and engine mounts.