The flight instructor reported that he and the pilot receiving instruction and flying the airplane were returning to land after a multi-engine training flight. As the airplane touched down on the runway, they heard a scraping noise. The airplane settled onto its belly and came to a stop. The pilots exited the airplane and saw that the airplane’s landing gear was not extended. The flight instructor reentered the airplane and saw that the landing gear circuit breaker was popped. The airplane sustained substantial damage to the lower fuselage. Functionality tests of the normal and emergency landing gear extension/retraction systems and landing gear configuration light did not reveal any anomalies. The flight instructor reported that there were no other preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.