One source lists this as the XF-84F; all others as the "second" YF-84F. In any case, it was created from a G-model, serial 51-1344. Whereas the "first" YF-84F aka YF-96A was created from E-model serial 49-2430 and the fuselage was not enlarged, this second prototype had the fuselage deepened 7" (some sources state 6") to accommodate a larger more powerful engine, the Wright YJ65-W-1, 7,200 lb. thrust rating. Initial flight tests revealed problems with this engine, but these problems were corrected for the production J65-W-3 engines used in the F-84F.
As with many experimental and service test aircraft, "1344" underwent modifications, one of the more interesting, for which no reason is currently known, being a false designation as 49-2430 (the serial number of the YF-96A/first YF-84F). Also, as may be seen by the photos herewith, it carried at one time or another both unbraced and braced canopies, and the test probe was located in at least two different positions.
Deepened fuselage, oval intake, swept wings and tail, bubble canopy with bracing tapes.