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Hi Greg,
While we're sure to disagree, that's not a problem. I wanted to make sure you were aware of some well-buried facts to fit into your own examination of the question.
You mention flight test reports. The first of these noted that the P-61 was never going to be a "world beater" and that performance was expected to degrade further with several necessary modifications. Wright Field instead informed Washington that everyone considered the P-61 to be an excellant night fighter.
Eglin tested the aircraft and reported that production should be stopped immediately, with efforts made to purchase Mosquito night fighters. Wright Field squashed the report and had a new set of officers write a gentler opinion. Even the revised report noted problems with the P-61's speed, ceiling, and endurance, all of which would be mitigated once the auto-gun-laying turret was developed and installed. The report ended with the recommendation that the turret should not be installed (losing the sole advantage) and external fuel tanks should be installed (further degrading top speed).
Several Wright Field reports noted incredible top speeds for the aircraft, each with an asterisk noting that the speed had not been measured and the figures were based on Northrop estimates. Later reports quietly admitted that the P-61's speed and altitude had not lived up to expectations, but the aircraft was still better than the P-70 or Beaufighter.
Post-war characteristic files gave indications of releasable statistics and classified statistics. The only aircraft where releasable statistics were better than the actual performance was the P-61. (Normally the files avoided informing potential enemies just how good an aircraft was; for the P-61 the releasable characteristics lied about how bad the aircraft was.)
There were very few who knew the truth behind the flyoff. Dick Leggett was the RAF pilot, and he wasn't even aware he'd been involved in a contest. The two (not three) American crewmen died in combat and weren't available for post-war interviews, but the squadron history noted that the American pilots were violently angry that there was to be an attempt to replace their P-61s with Mosquitos. Only three of their crews were considered gifted enough to defend the P-61's (and the squadron's) honor - they drew lots to see who would champion their cause. Compare this with Leggett's instruction to go fly with the P-61's and note his impressions of it's capabilities. These impressions were then written up by the RAF squadron CO to praise the the P-61 as an excellant night fighter.
All this happened after the Brits had to reject the AAF's request for NF.30s to re-equip the US night fighters. It didn't matter who won the flyoff - there were not going to be enough Mosquitos to meet America's needs. (One wonders what would have happened if Eglin's recommendations had been followed more than a year before.) Vandenburg subsequently wrote a report to Washington noting that the P-61 was the "best available" night fighter for his forces -- of course it was, the Mosquito was NOT available. Vandenburg and Spaatz still wanted Mosquitos, but dealt with the aircraft they were provided.
I was in contact with the two AAF officers sent from Washington to run the flyoff. Henry Viccellio died before we got too far into the discussion, but Winston Kraatz was certain that the Mosquito was still the better aircraft.
I ask myself how I would have determined which aircraft was the best night fighter. Would I have run a daylight dogfight with the two aircraft approaching each other from opposite sides of the field to see who could get on the other's tail? Amazing maneuverability was certainly the P-61's strong suit, but not a widely used tactic for night fighters. Would I have run the test above 20,000 feet, where nearly all enemy bombers were able to escape the Black Widow? I find it significant that that was the chosen altitude to end the tests. Would I have placed a target miles away and raced the two aircraft t see who got there first? Again, speed was certainly not factored into the tests. The fly-off had one purpose - to reenforce the confidence of the American crews in their aircraft so that they might fight to the limits of their aircraft's abilities. The test certainly accomplished that, but it did nothing to determine the best night fighter.
(Incidently, the NF.30 was already in production, but deliveries had been delayed due to critical cracks in their flame dampning exhaust manifolds. Coincidentally, the P-61s were being flown without flame dampners. Despite complaints that the Black Widow's exhaust flames were too bright and could be seen from miles away, the dampners further reduced the aircraft's speed and weren't used. How much easier is it to avoid interception from an aircraft that you can see coming?)
Anyhow, that's a small part of how I came to my conclusions. I've been researching America's search for a night fighter since I first hit the National Archives in 1972, and there are still many files I hope to dig through when Covid allows. With luck, I'll be able to release a small monograph on the subject next year...
Cheers,
Dana