On completion of a rest period, the pilot flew the aircraft to the first destination of an afternoon bank run schedule. Pre-landing checks were completed prior to turning onto final and the landing gear was confirmed to be down and locked. Shortly after touchdown, the nose of the aircraft contacted the runway. The nose landing gear was fully retracted and the nose gear doors were closed. An inspection of the aircraft immediately following the accident revealed that the main landing gear doors were partially open and the side braces were unlocked. When electrical power was supplied the braces locked into position and the doors closed. The aircraft was then raised and the nose landing gear was observed to cycle to the down and locked position. No defect was found which could have contributed to the accident. Evidence indicated that it was likely that early in the landing roll the pilot had inadvertently selected the gear up.