The pilot reported that the three gear position indicator lights were green as he commenced the take-off roll. At about 40 knots the gear commenced to retract and the aircraft slid to a halt with all wheels retracted and the position lights indicating that the gear was up and locked. No fault was subsequently found with the landing gear system which might have contributed to the accident. The pilot had not checked the position of the gear selector lever during his pre-flight and pre-start checks. The sequence of events during the attempted take-off was consistent with the selector being in the up position. The gear system includes an anti-retraction valve which prevents inadvertent retraction while the weight of the aircraft is on the mainwheels. When the aircraft had reached about 40 knots, there was evidently sufficient lift being developed to allow the anti-retraction valve to close and the gear to retract.