I need your help.
A few days ago, I plucked a gem from the local library called "MEN in the AIR." A bit of a chauvinistic but not entirely inaccurate title, this book is a collection of short aviation stories from all over prior to the book's 1990 publish date. However, one of the stories left me puzzled as to the aircraft type itself despite significant hints. I'm a "commercial aviation" guy, so the hints in this story may only be glaringly obvious to you sharp military enthusiasts.
The short story I am confused about is titled "The Experimental Flight of the US-XP." I thought at first that it was an X-15 flight, however the X-15 does not appear to be bomb-bay accessed in flight before release from the mother-ship, so I went back to the drawing table and hit upon one of the later X-1 flights. However, Google in all of it's collective intelligence has left me high and dry on this one with searches for "US-XP" and related articles to pioneer high-altitude flight.
In this story, the bomber is not identified but it's loud and has a personnel-accessible bomb-bay (Im thinking B-29 variant?) where the test aircraft is affixed before drop. The pilot of the bomber mother-ship was Midge Filene, copilot's name was "Begley." The pilot of the bomb-bay accessed "US-XP" was "Col. Duke" who had performed 5 drops prior in this ship, with this story reflecting his 6th. Duke tells the story from the first person, and describes US-XP as not being the "latest" testbed used for the US Air Force. 5 or 6 later prototypes had been flown, but US-XP was not junked as it evidently still had a thing or two to teach engineers on the ground.
US-XP was dropped from the bomber and fell about 1,000' before accelerating skyward to somewhere not far above 80,000' (another clue that it wasn't an X-15). Before nosing back forward at apogee, pilot "Col. Duke" turned hard one more time to get once last glance of space before turning back.. the story then switches to the chase plane pilot below (Matt) who realized things were amiss and worked to talk the US-XP home. Evidently when Col. Duke turned his head, he inadvertently pulled his oxygen tube loose and almost lost complete consciousness. The chase plane ended up landing empty (due to the extra time required to talk his friend down), followed by a safe landing by Col. Duke in US-XP at Muroc/Edwards.
There's all the clues I was able to extract--what flight/aircraft was this?