Strong winds in the upper troposphere were measured regularly in the 1930's by early versions of radiosonde balloons, but their true significance only became clear in Word War II. Allied bombers flying at 7.6 km over Germany in January 1945 encountered a tailwind of 66 m/s. They were covering the ground at 119 m/s, and reached their target early. On their westwards return, their ground speed dropped to as low as 26 m/s (94 km/h). Another group also met headwinds of 53 m/s in March 1945 at the same height, and the planes’ slow speed led to heavy losses from anti-aircraft fire.
The word ‘Strahlströmung’, meaning jet stream, was first used by the German meteorologist Seilkopf in 1939. But the phenomenon was not properly appreciated by the English until 1943, when a flight of their bombers ran out of fuel over occupied France against a wind of 54 m/s, and by the Americans not until late 1944, when their B-29 bombers faced extreme winds over Japan, where the winter jet stream is usually stronger over Europe. Carl-Gustaf Rossby was the key meteorologist advising the US Air Force (1).
(1) Phillips, N.A. 1998. Carl-Gustaf Rossby: his times, personality and actions. Bull. Amer. Meteor. Soc., 79, 1097-12.