On January 30, 2009, approximately 0810 central standard time, a twin-engine Cessna 421C, N345JB, was substantially damaged during a forced landing following the loss of power on both engines during initial takeoff. The airline transport pilot and the two passengers on board sustained minor injuries during the forced landing. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed for the Title 14 Code of Federal Regulations Part (CFR) 91 personal flight. The airplane departed Colonel James Jabara Airport (AAO), Wichita, Kansas, at 0808 with the intended destination of Millard Airport (MLE), Omaha, Nebraska. According to the pilot, he landed the previous night with 100 gallons of fuel on board the airplane. The pilot made a request to line personnel to add an additional 40 gallons of fuel to each tank. The morning of the accident, the pilot performed a normal pre-flight, loaded the passengers on the airplane, and taxied for takeoff. The pilot checked the engine instruments, magnetos, and propellers prior to takeoff. During takeoff, the pilot noted 2,800 rpm, 39 inches of manifold pressure, and all engine instruments were "in the green." While climbing to 3,000 feet mean sea level, the airplane's engines began to lose power. The pilot noted propeller rpms were still 2,800 but felt as though no "power" was being produced by the engines; the pilot coordinated for an emergency return. While attempting to troubleshoot the malfunction, the pilot assessed that he could not return to the airport and elected to perform a forced landing. The pilot maneuvered around transmission lines and landed gear up in an open field. The pilot called for emergency services on a cell phone and attended to his passengers. An examination of the airplane revealed that the fuel tanks contained what appeared to be a mixture of 100 low-lead (100LL) and Jet-A fuel. A line person employed by the fixed base operator (FBO), reported to FAA inspectors that he had fueled the accident airplane with 80 gallons of Jet-A. Of note, two Piper PA-46s based at AAO were configured with Pratt & Whitney PT6A turbo-prop engines through a Supplemental Type Certificarte (STC) which does not require a modification of the fuel filler opening. The STC modified airplanes operate with a smaller fuel filler opening than required by Title 14 CFR 23.973. The STC modified airplanes fuel filler openings have placards notifying of the use of Jet-A fuel in accordance with 14 CFR 23.1557. The accident airplane's fuel filler openings were modified in accordance with airworthiness directive 87-21-02 R1 to prevent the "flattened" Jet-A fuel nozzles from entering the fuel filler ports. The placard near the right main fuel tank filler cap was worn and unreadable. The placard near the left main fuel tank filler cap was legible and was in accordance with the FAA approved airplane flight manual. The FBO's Jet-A fuel truck had the "flattened" nozzle, so the line personnel discovered that the STC modified PA-46s could be refueled without the adapter by rotating the nozzle and dispensing fuel at a reduced pressure. This method became the normal way for the accident line person to refuel the two modified PA-46 airplanes, so the line person reported that he incorrectly thought the accident airplane required Jet-A fuel despite having refueled the accident airplane several times previously.