Drop Tanks

[NARSES2]

Interesting photograph - not sure if you got hit in the fuel tank you'd have time to jetison it, but interesting idea and shows how the threat of fire was on everyones mind in WWI particularly

[Mossie]

Found a few snippets on the Flight archive

This one dated 1927 documents a record flight by the RAF to fly a Hawker Horsely from the UK to India.  External tanks were considered, but it was decided to keep the aircraft as clean as possible.  There's no suggestion that the tanks were jettisonable, but it does show that idea of external tanks was considered but that drag reared it's ugly head in early aircraft.
http://www.flightglobal.com/pdfarchive/view/1927/1927%20-%200360.html

This series of letters to the editor dated 1930 talks about droppable tanks in the event of an emergency to prevent fire.  The replies suggest that it's a bad idea.  It's nothing concrete, and just seems to be an ordinary guy contemplating an idea, but it seems it was being thought about at least:
http://www.flightglobal.com/pdfarchive/view/1930/untitled0%20-%200278.html?search=correspondence+2281 (see 2281)
http://www.flightglobal.com/pdfarchive/view/1930/untitled0%20-%200349.html?search=correspondence+2290 (see 2290)
http://www.flightglobal.com/pdfarchive/view/1930/untitled0%20-%200526.html?search=drop+tanks (see 2297)

[rickshaw]

The Japanese were using drop tanks in China in the mid-1930s on their Ki-27 fighters and other aircraft.