The pilot reported that during the initial approach, he was given vectors from air traffic control (ATC) that took him about 15 miles away from the airport. As he was vectored back toward the airport, ATC notified him that he was fifth in sequence to land and was asked to decrease airspeed. About 5 miles from the airport, ATC turned multiple airplanes in front of him, and he continued to maintain visual contact with each airplane. The spacing was tight with airplane in front of him and he continued to watch it as it slowly exited the runway. While on short final, he forgot to complete the before landing checklist, which resulted in him not extending the landing gear. The airplane landed on its belly and came to rest upright on the runway, which resulted in substantial damage to the bottom fuselage. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.