The flight instructor in the retractable-landing gear, multiengine airplane reported that, while performing aerial survey training for a newly hired pilot, the airplane was slowed to the mission airspeed of 117 knots, which meant reduced power and the use of more than 15° of flaps. He added that he then pulled the landing gear warning circuit breaker to silence the landing gear warning horn to ensure clear communication but that he did not pull the landing gear motor circuit breaker. The pilot receiving instruction reported that he did not know that the instructor had pulled the landing gear warning circuit breaker. The instructor further reported that, while returning to the departure airport, the landing checklist was "accomplished" 15 to 20 miles from the airport. He saw that the pilot was "falling behind on power and descent management" and that the airplane was high and fast for the approach. The instructor advised the pilot to correct but then began to assist the pilot when it "was taking longer [than anticipated.]" The airplane decelerated into the white airspeed arc, and the instructor moved the landing gear selector switch into the down position and added full flaps. He added that, because the flaps were added shortly after the landing gear selector switch was put into the down position, he could not tell by the flight characteristics that the landing gear had not extended. Neither he nor the pilot noticed that there was no green down-and-locked position light indicator. The airplane landed with the landing gear retracted. The airplane sustained substantial damage to the fuselage and both engines. The instructor added that, while configuring the airplane on the runway, he observed that the landing gear warning horn and landing gear motor circuit breakers were tripped. The director of maintenance reported that, during a postaccident examination, the airplane was lifted onto jacks and an emergency gear extension and three electrical landing gear retractions were conducted, and no defects were found. In a photograph provided by the flight instructor, the landing gear warning horn and landing gear motor circuit breakers are right next to each other. Despite the instructor's statement that he did not pull the landing gear motor switch, given the proximity of the breakers to each other, it is likely that the pilot inadvertently pulled the landing gear motor switch when he pulled the landing gear warning horn switch, which prevented the gear from extending. The airplane manufacturer's Pilot's Operating Handbook contained a note in the section titled, "Landing Gear Warning Horn," which states, Do not pull landing gear warning circuit breaker to silence horn as this turns off the landing gear control relay, thus the landing gear cannot be retracted.