A General Aviation pilot reported a gear up landing.

Narrative

Approaching ZZZ from the NW I began making standard radio calls. I descended to pattern altitude and entered the upwind for RWY XX. As I neared the crosswind turn; I noticed a high-wing aircraft at patten altitude; flying east; just off the departure end of XX. I began my turn to crosswind leg early to avoid the aircraft. I then established on a left downwind for XX. At the midfield downwind; I noticed a tractor mowing the runway and made a mental note to land beyond the tractor. I extended flaps to 15 and rechecked the tractor's location. I then added full flaps and began the base leg turn. Turning to final approach I saw the tractor move to the south side of the runway and stop. I then focused on my touchdown point. Touchdown was on center line and smooth until the propeller blades contacted the turf; followed by the aircraft's belly. Deceleration was rapid and the aircraft skidded off the south side of the runway and stopped. I shut down the aircraft and exited. Human factors. I believe the two minor distractions were enough to break my normal procedure pattern. Expectation bias was also a factor as I have flown the aircraft for over 20 years and landed at this airport many times. I failed to verify the landing gear position; something I normally do twice in the pattern. Going forward; I will try to always double verify the landing gear and use a paper or electronic landing checklist.