The pilot made a fairly long taxi from the ramp to the end of the runway and then started his takeoff roll with the parking brake still partially set. The brakes became very hot and the component parts expanded. After rolling almost 3,000 feet the brakes tightened up to the extent that both main gear tires started leaving black skid marks which continued until the aircraft ran off the end of the runway without obtaining flying speed, a distance of 3,168 feet. It was noted, through a design feature of the aircraft, that the parking brake handle could be placed in the released position without the brake being released. However, this would give the appearance that it had been released. The main gear tires were worn almost slick during this attempted takeoff. Both main gear brakes had been overheated during this time to the extent that they were destroyed. The aircraft was destroyed by collision with trees, rocks and a fire hydrant at the bottom of a hill off the end of the runway.