Good question astir.. .I too look forward to the replies from the knowledgable among us.. .From memory, the Mitsubishi Zero pilots were using them well before Pearl Harbour when training for long range missions (Saburo Sakai/Martin Caidin - Samurai).. .Spitfires had the supposedly jettisonable slipper tanks and the Germans were using drop tanks over England by 1941(I think..can't recall sources).. .The Voison 'pusher' circa 1916 had beautifully streamlined drop tank styled fuel tanks but they were permanently attached under the top wings.