The pilot of the retractable landing gear-equipped airplane reported that, he failed to extend the landing gear and the airplane landed with the landing gear retracted. The airplane skid off the runway to the right and came to rest in the grass adjacent to the runway. The airplane sustained substantial damage to the fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The owner of the airplane reported that the pilot had not been authorized to fly the airplane the day of the accident. Local law enforcement officers conducted a field sobriety test and concluded that the pilot was impaired to the point that he was unsafe to operate an airplane. Subsequently, he was arrested for operating an aircraft while under the influence.