B-17A (Y1B-17A)
The aircraft that became the sole Y1B-17A was originally ordered as a static test bed. However, when one of the Y1B-17s survived an inadvertent violent spin during a flight in a thunderhead, Air Corps leaders decided that the plane was exceptionally robust and that there would be no need for static testing. Instead, it was used as a testbed for engine types. After studying a variety of configurations, use of a bottom-mounted turbo-supercharger was settled on. This turbo-supercharge became standard on the first production model, and allowed it to fly higher and faster than the Y1B-17. When testing was complete the Y1B-17A was renamed the B-17A.
Read more about this topic: Boeing B-17 Flying Fortress Variants