And another clue. Thanks; Ernie P.
What warbird do I describe?
Clues:
1. This aircraft was the most important of its type, at the time, for its military.
2. But it was also the only one of its type, at the time, in its military.
3. It was developed for the military, but much of the testing was conducted by a civil aviation business.
4. This aircraft first flew in 1935.
5. This aircraft set a number of distance/speed records.
6. One of these aircraft set a distance record, flying over 5,000 miles from start point.