This is one iteration of the two XP-51B's (built from NA-91 Mustangs}.
"Notice that there's NO carb air intake above OR below the prop spinner. This one had a "slit" (for lack of a better term) in the lower fuselage (actually the underside of the wing) to feed air to the updraft carburetor on the Merlin.
This "straight" coolant housing ("doghouse") scoop mouth, which also showed up on the first few production P-51Bs, was discovered in wind-tunnel studies of the actual 2nd XP-51B prototype (minus the extension/wingtips, so it'd fit into the wind tunnel that was available - in April, 1943 at Ames Aeronautical Labs), was discovered to be what was causing "duct rumble," that showed up at higher speeds in the Merlin prototypes and early P-51Bs.
Reshaping/recontouring the mouth of the doghouse "cured" that rumble. I've got a copy (not an original) of the report, replete with photos, etc.
Anyway, the "correction" was made to the production P-51Bs (and the two Prototypes, I guess, since they were still flying them).
That "slick" nose was carried through on the XP-51J , which had its carb air intake in the front part of the doghouse, maybe...don't quote me on that".
Quote: Tom Griffith
Photos: Paul Varga Collection via Richard Miller via NAA
Below: Instrument Panel Arrangement - XP-51B
Below: P-51B Firewall Forward Innards
Below: P-51B model nearing completion in Inglewood plant
Paul Varga Collection via Richard Miller via NAA by
Historic Aircraft, on Flickr
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