We will be skipping most of the P-51 history lesson and only delve into the
pertinent aspects of development and production.
We will begin in March 1940, when NAA's chief designer, Edgar Schmued was approached by NAA's President, Dutch Kindelberger and asked, "Ed, do we want to build P-40s here?" Well, Schmued had been long awaiting a question like this. "Well, Dutch, don't let us build an obsolete airplane, let's build a new one. We can design and build a better one."
Kindelberger and VP Lee Atwood sold their idea to the British and the race was on. NAA promised the British a fighter flyable in 120 days. That's the time it would have taken to tool up for building the P-40 at NAA. Deliveries to England were to begin in January of 1941.
Work began in April. The team consisted of chief engineer Ray Rice, chief designer Edward Schmued, chief aerodynamicicst Ed Horky, team members Stan Smithson and Larry Waite.
NAA was required as part of the negotiations with Britian's Sir Henry Self, to obtain the wind tunnel data of the Curtiss XP-46 program. Lee Atwood did so, purchasing the data, but according to Atwood the data was never used in the design of NA-73 - although when you look at the two designs, there are similarities.
The first prototype was designated NA-73X. NA-73X was actually ready in 102 days but was less the Allison V-12. NAA waited on Allison's doorstep until the V-12 was ready. When they finally got the Allison, it had been modified from the drawings and
NAA had to build new motor mounts. The prototype also had no armament yet and used
some T-6 parts.
The P-51 was designed around the Allison V-1710-39 V-12 and a 5-foot 10-inch 140-pound pilot creating a very sleek and narrow fuselage. The P-40, which also used the Allison was originally designed for an air-cooled radial engine resulting in a large frontal area of drag. NAA decided to use a brand new wing design by NACA called the NACA-23 Series Airfoil. It was a low drag laminar flow airfoil. This was a very bold step especially considering the timeline promised. There were tests and data but no real proof the new wing design would work as well as the design tests predicted.
NAA's head aerodynamicist, Ed Horkey, adapted the NACA profile for the P-51. The basics of laminar flow is the thickest part of the wing, which is normally about 20-25 percent back from the leading edge, is moved further back to about 50 percent of the wing cord. Both the top and bottom of the wing were evenly contoured, somewhat like a copy of the top was put on the bottom.
The radiator would be in the belly of the aircraft with its own intake scoop. The design of the radiator scoop and exit door was supposed to not interfere with the wing and fuselage aerodynamics. It if did, that would cause turbulence and then drag. It was later found out (in wind tunnel testing) that the hot air from the radiator actually created thrust as the hot air exited at a greater velocity than it entered.
The stressed aluminum skin would have all flush riveting or screws for speed. Large self-sealing fuel tanks would be installed in the wings holding nearly double the fuel as the Spitfire - 170 gallons. The fuselage tank would come later in production. The Brittish specs included heavy armament of 8 guns. NAA planned to use four .50 cal and four .30 cal guns. Two .50 cal guns would be installed in the nose with syncro firing through the propeller, the other two in the wings along side the .30 cal guns. The wing mounted guns were staggered for a better fit.
The landing gear was designed with a wide-track of 12 feet and would fold
inward. In contrast, the ME-109 folds outward and has a much narrower track
resulting in difficult handling on the ground rolls. The Spitfire and P-40
also had narrower tracks. The P-51 will incorporate closing inner gear doors
while the gear was out minimizing drag with the gear extended. The tail
wheel is also fully retractable
and can be steered by the rudder (limited) or free-swinging.
The engine installed was the Allison V-1710-F3R liquid-cooled V-12 rated at 1,100 horsepower turning a 3-bladed Curtis electric propeller.
Every effort was made to get the highest performance and then to make the P-51 easy to manufacture. The desing team at NAA, with Schmued at the head, would accomplish this goal.
NA-73X was all metal skinned except the rudder and the elevators. The guns were never fitted. In March of 1941, NA-73X was torn down a bit for landing gear tests. The final fate of NA-73X is unknown and there is no consensus on its demise.
Testing of NA-73X went very well and production started as soon as NAA could going. The only gliche in the testing was a force landing which put the prototype on its back in a plowed field. Cause was pilot error - fuel starvation (there has been speculation that it could have been a problem with the induction intake but I've only read that in one printed source). Repairs were made and testing continued as production of the Mustang Is began.