Who's going to be the first to point out in the answer to question 1, the famous four months taken was trying to 'design' a way to fit the engine to an already existing airframe, NOT designing a 'plane from scratch?
That's not the case. The P-51 was designed from scratch, principally beecause North American didn't want to get lumbered producing Curtiss fighters for the British under licence.
Here's a brief timeline:
January 1940: British Purchasing Commission (BPC) visits US looking to buy a fighter. This is where the suggestion that North American build the Curtiss P-40 originates. North American reviews the British proposal and in turn proposes building a superior new design instead.
April 24 1940: The BPC accepts the preliminary design of the NA-73X.
May 4 1940: British approve design proposal for the NA-73X
May 29 1940: British order 320 NA-73 aircraft with the name 'Mustang'.
September 9 1940: The first NA-73 aircraft is rolled out of the North American plant, without an engine.
October 7 1940: Allison engine arrives at North American
October 26 1940: First flight of NA-73 at Mines Field. Vance Breese is the US test pilot.
Though the time from the initial British enquiry to the approval of the design was indeed four months, it was ten months to the first flight. It is not until a year later that AG346, actually the second Mustang I, arrives in the UK.
October 24 1941: AG346 arrives in UK. It is fitted with British wireless and guns and ready to fly at the end of the month. It made a creditable 382 mph at 14,000 feet.
Testing continued at the A&AEE, Boscombe Down for several months.
April 1942: No. 2 Squadron of the short lived Army Co-operation Command is the first unit to receive the Mustang I for operational use.
May 10 1942: No. 2 Squadron raid an airfield in France, the first operational use of the P-51/Mustang.
For the first two years of its life, the P-51/Mustang was powered by the Allison engine for which it had been designed. It was a good aeroplane, but its performance at altitude was limited by the performance of its engine. It was the British who first thought they could do better, and this brings us to the question of engines.
July 14 1942: Rolls-Royce make a feasibility study for installing a Merlin 61 engine in the Mustang airframe.
July 25 1942: Contract issued to North American to convert two lend-lease P-51s to XP-78s using Packard Merlin engines.
August 1942: In simultaneous development on both sides of the Atlantic, North American make a prototype installation of the Packard Merlin engine and five Mustang Is are sent to Rolls-Royce for the development of a Merlin version, 'Mustang X'.
September 1942: The Americans re-designate their Packard Merlin powered XP-78s as XP-51Bs (you can see where that's going with US designations).
October 14 1942: The first Merlin powered Mustang makes a first flight in England. In tests this aircraft made 422 mph at 22,000 feet.
November 30 1942: The first Packard Merlin powered XP-51B flies in the US.
Th fruits of this development began to materialise on both sides of the Atlantic almost simultaneously. However, it was not rational to have Rolls-Royce, a hard pressed British aero engine manufacturer, continue the process rather that the aircraft's designer and builder.
January 19 1943: US data on the XP-51B is sent to the UK. The decision is taken not to modify existing Mustang Is in the UK but to wait for US production of the P-51B.
There were production problems, notably the accumulation of Mustang airframes awaiting engines from Packard. Nonetheless, after three years, the rest is history.
Obviously, there are some other more or less salient points I have excluded from the timeline for the preservation of my sanity and brevity :smiling3:
