BE18 pilot reports requesting avgas from the fueler but not observing the actual fueling. During takeoff the engines began to backfire and eventually quit when the throttles were retarded. The tanks had been topped off with jet fuel.

Narrative

I was to fly a Beech-18 aircraft with radial engines carrying about 1;000 LBS of newspapers. After doing a preflight and run up; the plane was loaded with the cargo. After loading I had to reposition the aircraft from the cargo area to the southwest part of the airport for fueling. At the facility I went in and told the person in charge that I needed the inboard tanks of the aircraft topped off with avgas. The person never asked me to fill in any fueling papers. I then went to the restroom; planning to return and supervise the fueling. When I returned; the person said that the plane was already fueled and I did not need to sign any paperwork as the company I was flying for had an account with them. I returned to the plane and did another preflight along with draining and checking the fuel. I got into the aircraft; checked the fuel tank gauges and they showed full. I received my ATC clearance; started engines and taxied out. After getting to the end of the runway; I did another complete engine run up and system check. All looked good. I asked for and received takeoff clearance. On the runway I slowly added power and checked all engine readings. All checked good. I then added full takeoff power and continued my takeoff run. At about 40 KTS both engines started to backfire. I reduced power and both engines quit at the same time. I told the Tower that I was having problems and would try restarting the engines. Nothing worked. I had to be towed off the runway and to the company's parking area. I found out that the fuelers had put in jet fuel instead of avgas; thus causing the problem.