The flight instructor stated that during a simulated one engine inoperative approach and landing, the pilot receiving instruction chose to leave the landing gear retracted until the landing was assured. The pilot receiving instruction turned the airplane onto a short base leg before final, increased the mixture and propeller control settings, and increased the airplane bank angle so as not to overshoot the final approach course. The flight instructor saw that the airplane airspeed was decreasing and told the pilot receiving instruction that they were low. The pilot receiving instruction increased airplane pitch, which resulted in a decrease in airspeed, a loss of altitude, and a rolling motion. The flight instructor leveled the wings and reduced pitch to regain airplane control and lift. When the airplane was over the runway threshold, the flight instructor did not remember that the landing gear was still retracted and he did not complete the final landing checklist, which resulted in a landing with the landing gear retracted. The airplane sustained substantial damage to the underside fuselage longerons. There was no mechanical malfunction/failure of the airplane or system that would have precluded normal operations.