Your photo above shows the first of the two airframes that were converted to XP-61E standard. The 0.50 cal Brownings in the nose were configured differently between the first and second prototypes. The first had a "box" arrangement, while the second had a nearly flat, horizontal spread. There were several other detail differences, including the canopy, instruments/radios, and so on. The second did not last long, being wrecked on take-off on an early test flight by a hot-shot USAAF pilot, who apparently ignored a lot of the preflight briefing, as he "knew better"...
This airframe in your photo was the one that became the first XF-15. The operational F-15A versions were all based on the turbo-charged P-61C airframe.