On October 31, 1996, about 1415 central standard time, a Piper PA-31-350, N120JD, registered to SOHO Air, Inc., lost power in both engines, shortly after takeoff from McComb-Pike County Airport, McComb, Mississippi, while on a 14 CFR Part 91 corporate flight. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft received minor damage and the airline transport-rated pilot and three passengers were not injured. The flight was originating at the time of the accident. The pilot stated that after landing at the airport, he requested that the lineman fuel the aircraft with 20 gallons of 100LL avgas in each main tank. This brought the fuel capacity to about 80 gallons. About 30 minutes after fueling he loaded the passengers, started the engines, and taxied to the runway where he made a normal engine run-up. After departure, while climbing through about 300 feet agl, one or both engines surged. At 400 feet agl, both engines lost power. He made a forced landing in a field. Records obtained from the aircraft fueling company which fueled N120JD just before takeoff shows the aircraft was fueled with 40 gallons of Jet-A fuel. The pilot had signed the receipt indicating the aircraft had been fueled with Jet-A fuel. The lineman stated he had fueled the aircraft with Jet-A fuel. He did not see the pilot check the fuel in wings before takeoff. Examination of the aircraft after the accident by FAA inspectors showed that the main fuel tanks and engine fuel strainers contained a mixture of 100LL avgas and Jet-A fuel. Examination of the fuel pump which was used to fuel N120JD showed the pump was not equipped with a nozzle designed to prevent misfueling of aircraft with Jet-A. N120JD did have restricted fuel ports to prevent misfueling.