The flight instructor reported that during the instructional flight he and the pilot receiving instruction had just completed a simulated single-engine approach to a landing. During this approach, the right engine throttle was reduced to idle to simulate the failed engine and the landing gear position warning horn circuit breaker was pulled to silence the horn. After landing without incident, the crew departed with the intention of practicing short field landings. The flight instructor reported being distracted by incoming traffic, having to execute a 360º turn for spacing, and not completing the before landing checks using the printed checklist. The landing gear warning horn was not audible due to the circuit breaker not being reset and the landing gear lever was never selected to the down position prior to touchdown. The airplane contacted the runway with the landing gear retracted resulting in substantial damage to the lower fuselage. The flight instructor reported that there were no preimpact mechanical malfunctions or failures of the airplane that would have precluded normal operation.