1935, and then the search was on for the most reliable machine gun, because prior to the Hurricane and the Spit the guns had been mounted on either side of the cockpit in biplanes so the pilot could clear stoppages by using the cocking handles. As you say, they settled on the Browning very quickly as the most reliable machine gun in the world. The one mistake I think they made was going with 8x .303 armament rather than .50. Six .50 cals would have contributed significantly to the Hurricane's and Spitfire's ability to knock down German aircraft during the Battle of Britain. The British adopted the Hispano Suiza 20mm cannon which was eventually made very reliable, but I still think the Browning M2 was the best balance for air combat.
An interesting and little known fact was that Air Marshall Hugh C. T. Dowding, called "Stuffy" by his pilots for his remarkable shyness, moved from the committee which oversaw the creation of the specifications for the fast monoplane fighters, to the committee responsible for the search for Radar and then setting up the Radar and Observer Corps reporting network, and the massive telephone network which linked everything with the Control rooms, to being C in C of Fighter Command in 1939. If there's one person who deserves credit for Britain's survival it's Stuffy Dowding. During all of this the RAF bureaucracy was treating him like dirt, changing his orders on zero notice and trying to retire him (early). He was told to clear out his desk on one days notice in November 1940 and was replaced by Sholto Douglas. His pilots who were initially put off by his shyness came roaring to complain about the shabby treatment, but bureaucracy ruled and Stuffy went off to the US where he was head of the Fighter Purchasing Commission.
Britain approached North American in April 1940 to build Curtiss P-40's, but Dutch Kindleburger thought the Curtiss was a hunk of junk and persuaded the Brits he could design and build a much better aircraft and produce the first prototype in 120 days. He did it in 119, but had to roll the aircraft out on AT-6 Texan wheels because they hadn't been able to get the right ones made.
As you mention the original a/c was Allison powered, and was apparently very sweet to fly, but suffered from the Allison's fall off in power at altitudes above 12,000 feet. A ground attack version designated the A-10 Apache was supplied to American squadrons which didn't require superior high altitude performance.
The first lot of Allison engined P-51's arrived in Britain and examples were sent to the Royal Aircraft Establishment at Farnborough to be wrung out, and as you mention, the Brits were disquieted by the lack of high altitude performance since they were defending against the Luftwaffe from 16,000 feet up to 30,000. The sizes of the two engines were similar so the RAE cobbled together engine bearers and other plumbing and put a Merlin in it, and it went from very sweet to a world beater, but a bit of a bear to handle on the ground. The Brits never seemed to mind that. It was they who pioneered landing the Corsair on carriers after the US Navy decided it was a little too hairy to use that way. US Corsairs until late 44 or early 45 were operated from island landing strips only.
Then the Brits decided that they didn't like the rearward view on the Mustang, and put a Spitfire "hood" (windscreen and sliding canopy) on the Mustang which gave North American the idea to create a bubble canopy which made enormous difference in vision for the pilot. Britain had its own design for an emergency fighter, the Miles M-20 which did have a bubble canopy, and the first that I know of unless we include the P-38 which was an odd sort of bubble hood with vertically sliding windows, not a whole unit which slid rearward.
American pilots who were by 1942 arriving in Britain were hearing about the Mustang (British aircraft all have names while American aircraft mostly were identified by numbers. Even the Curtiss Warhawk was referred to as the P-40, it was Britain who named the variants Tomahawk, Kittyhawk, Warhawk etc) and went down to Farnborough to try out the Mustang, and then sent letters back to North American and the Air Force command demanding the Merlin engined Mustang.
With regard to names, my Dad flew Douglas DB-7's which the British called Bostons or Havocs depending on their role. It was a very popular aircraft with the Brits and a great attack bomber, although my Dad lost his heart to the de Havilland Mosquito which he flew (though not on operations) as often as he could.
Britain wouldn't have survived without American machine tools, and American Stromberg and Bendix carburettors (their spelling) and American made instruments. Still, they designed some truly superb aircraft and were willing to take chances on aircraft that others ruled out without even trying the idea. The concept of the Mosquito came partly out of de Havilland's experience before the war with plywood monocoque airliners. While in development was called Freeman's Folly because Sir Wilfred Freeman was the only one in the Air Ministry who would support it. An unarmed bomber?? Are you joking?? But it worked out brilliantly and became a remarkable multi role aircraft as a bomber, a fighter, an attack fighter bomber, a night fighter and a fast reconnaissance aircraft. Top speed of the prototype was 410 mph in 1940 roughly the same as the P-38 which was first to break 400.
The Brits were very big on the Schneider Trophy races for float planes, and Rolls Royce designed a 36 litre engine based somewhat on the Curtiss engine which powered the American Schneider Trophy winner piloted by Jimmy Doolittle. The engine took a lot of coaxing and special handling but eventually drove Reginald Mitchell's design, the Supermarine S-6 and S-6b to the second and third victories (first one was the Napier Lion powered S-5) which won them the Schneider Trophy outright. Both the Trophy and the Supermarine S6b are in the Science Museum in London. The S-6b is still in its original paint. The S-6b won the competition at 340mph, and then a couple of weeks later set a World Speed Record considerably in excess of any land plane at the time of 407 mph.
The R type engine derived from the Rolls Royce Buzzard wasn't very good in its non-race configuration, but Rolls scaled it down to a 27 litre version originally called the PV (Private Venture) 12 which went on to become the Merlin. Then using what they had learned through the races and with the Merlin, they built a new 36 litre engine called the Griffon which went on to power later Marks of Spitfire. The wonder of it all was the Spitfire accepted whatever the designers could throw at it. Single Stage single speed Supercharger giving way to Two Stage two speed Supercharger with Intercooler 9 inches longer, giving way to Griffon which was about a foot and a half longer. The most noticeable change was in the tail plane which had to be enlarged as the number of propeller blades rose from 2 to 3 to 4 to 5. They even had bubble canopy variants of the Spit. Power started at 975 hp then 1030 hp for the Supermarine Spitfire Mk 1 rising to 2050 hp for the Mk 21 with a Griffon Engine. The all up weight of the Spitfire (4810 lbs empty Mk 1) went from 6317 lbs fully loaded up to 9200 lbs in the Mk 22 fully loaded.
For all the brilliance of the Daimler Benz family of engines, they never achieved the flexibility of the Rolls Royce design nor the doubling of available horse power through the war.
Packard Merlins were prized by the Brits because of the tweaks that Packard introduced, and most went into Lancaster bombers.
The icing problem with the guns was addressed by having adhesive cloth patches which covered the gun ports until shot away by the first burst. They also thinned the grease used with petrol which helped, but as you note a duct from the radiator was also used to heat the breeches of the guns.
Henry Ford was a cantankerous SOB, and as you point out stridently anti-British and pro-Nazi. They (FDR;s Administration) had to brow-beat him into building B-24 Liberators, and build him a free factory to do it with.
Thanks Paul for allowing me to indulge my passion for prop-driven aircraft.