Just looking at a photo of NX19901 in Flight of February 16, 1939 (interesting to see that date protocol in those days). The caption reads:
ON TEST: The new Boeing 307 Stratoliner flying at 11,000ft. with its two starboard Wright Cyclones stopped and the airscrews feathered. During this flight the machine had an all-up weight of 45,000 lb. The way in which the rudder is being held over to counteract the drag of the stopped engines is noticeable, but more interesting is the white leading edge of the tailplane. This is a paint coating to record the direction of the flow of fuel in dumping tests during which coloured water was used instead of petrol.