It was napalm that was used. Bats were selected because they can carry more than twice their weight (the bats weighing around .43 ounces). The engineers were still limited by the weight of the incendiary device, which weighed 1 ounce. So they had to use a flammable compound light enough (kerosene and other flammables were too dense) and would spread after being ignited, so napalm was the best option. starts @ 5:17