BE35 Flight Instructor elaborated the events leading up to a gear up landing.

Narrative

On final approach the pilot flying reached over to extend the landing gear. He successfully accomplished the task; based on the fact that I saw the gear handle move into downward position; heard the gear system extending; felt the aircraft slow noticeably; and saw the three green lights; associated with the landing gear; illuminate. As we continued our approach; the pilot extended the flaps in further preparation for landing; and we prepared for a normal landing. As we flared out for landing; I heard a loud noise; and I assumed the landing gear had collapsed. I called 'my airplane;' and took control. I cannot say whether the other pilot acknowledged the 'my airplane' call; but I used the rudder and ailerons to maintain directional control as best I could. As we skidded to a stop; smoke began to appear in the cockpit. I ordered everyone to leave their things and get out. Being by the only door; I held it and waited for everyone to exit before leaving the aircraft. It is my judgment that I did everything that was necessary; except to verify that the gear was indeed still extended on short final. In the future; I will be sure to triple check the three green indicators. As to the cause of the incident; I cannot say. I saw the pilot extend the gear; and I saw the green lights; indicating the gear was down and locked about 90 seconds before touchdown. Thankfully; no one was injured; and no damage to the airport was apparent.