The pilot reported that this was his third skydiving flight of the day and he performed a back taxi on the runway for takeoff. He further reported that as he rotated the airplane for takeoff, he heard a "steady" stall warning horn, the flight controls felt mushy, and the airplane would not climb. The pilot reported that he aborted the takeoff and applied max braking and reverse thrust, but the airplane overran the runway remaining. Subsequently, the landing gear collapsed and the airplane collided with a fence. The left and right wing sustained substantial damage. After the accident, the pilot reported that he observed the wing flaps in the fully retracted position. He reported that the flaps should have been set to 20 degrees for takeoff. The pilot reported that he forgot to set the flaps during the back taxi for takeoff because he was communicating with other airplanes near the airport. The pilot reported that he did not use a physical checklist, but he did run through a checklist mentally. The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.