Maybe a secret Project Manhigh, eight years before the declassified version?
http://en.wikipedia.org/wiki/Project_Manhigh
http://en.wikipedia.org/wiki/Project_Excelsior
Some of what the military did back then was amazing, even by today’s standards:
Project Excelsior was a series of high-altitude parachute jumps made by Colonel (then Captain) Joseph Kittinger of the United States Air Force (USAF) in 1959 and 1960 to test the Beaupre multi-stage parachute system. In one of these jumps Kittinger set world records for the highest parachute jump, the longest parachute drogue fall and the fastest speed by a human through the atmosphere, all of which still stand.
(emphasis mine)
As this could be done in 1955, why not something similar in 1947? In fact, something similar was done in 1931: http://en.wikipedia.org/wiki/Auguste_Piccard
On 27 May 1931, Auguste Piccard and Paul Kipfer took off from Augsburg, Germany, and reached a record altitude of 15,785 m (51,775 ft). During this flight, Piccard was able to gather substantial data on the upper atmosphere, as well as measure cosmic rays. On 18 August 1932, launched from Dübendorf, Switzerland, Piccard and Max Cosyns made a second record-breaking ascent to 16,200 m (53,152 ft). He ultimately made a total of twenty-seven balloon flights setting a final record of 23,000 m (72,177 ft).
Of course whatever the military might have been doing at the time must have had some kind of experiment or application well beyond what Auguste Piccard did. Maybe an attempt to use manned balloons as a precursor to the U2 which first flew in 1955 (as also suggested by IL)?
http://en.wikipedia.org/wiki/U2_spy_plane
