The flight instructor in a multi-engine, retractable landing gear-equipped airplane reported that, the pilot receiving instruction was on the controls, and reported feeling some fatigue and motion sickness due to light turbulence in the pattern. During their discussion they were distracted and did not complete the landing checklist and did not configure the airplane for landing. Subsequently, the airplane landed with the landing gear retracted.

The airplane sustained substantial damage to both engine gear boxes.

The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.