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Well, I see that some of the folks apparently get their history from fairy tale books, and are "commenting" like crazy on the P-51 posts in this forum. Back when I was a young man, there was a real chick-flick, called "Love Story." The theme song to it was sung by Andy Williams, and aside from being on the Top 100 Songs list for a number of weeks, it may have won some awards, too. The title and first line of the song was "Where Do I Begin?". That's fitting for what follows. Before I get cookin', please know that about 90% of the stuff that I grew up learning about the P-51 Mustang and its siblings, the A-36A and the F-6, was W-R-O-N-G! I've had to be educated - many times from folks who knew a lot more about the subject than I even thought I knew, and other times by reading many books and articles. This has all helped me immensely. I have also read a number of official USAAF books, as well as USAAF and North American Aviation documents. Furthermore, I have done some checking with archivists, etc. I'm working on my Doctorate in Mustangology, you might say! ...so here's where I begin: The Mustang (which it wasn't called until the RAF accepted it - and it was a British name at that - and, neither North American Aviation nor the USAAF/USAAC gave it an official name at that point in history, because it was not developed for any branch of the US military) was designed 100% by Americans, and engineering drawings, etc were all done by Americans as well. I'm talking about the gang at NAA. It *WAS* designed based on British specifications but the Brits did not develop, design nor draft any of the drawings that resulted in getting the protype aircraft built. True, they made “suggestions” and “contributions” throughout its development and operational history, many of which were incorporated into the production Mustangs as the War went on and on. NAA called the prototype "NA-73x" and the first samples provided to the RAF (the USAAF latched onto a couple of the samples, too) were called "NA-73." The first 380 production aircraft that the Brits called "Mustang Mk I" were also NA-73s. The next batch, 300 aircraft, was designated NA-83 by North American. Because there were no real differences between the NA-73 and NA-83, the 300 later aircraft too, were called “Mustang Mk I.” The only difference I’ve ever seen is that the -73s had rounded cross-section exhaust stacks, and the -83s had fish-tail ejector exhaust tips. After those 680 were built and supplied, the Brits wanted the aircraft to be armed with four 20 mm cannons, replacing the four .50 Cal Browning Machine Guns and the four.30 Cal BMGs. NAA came up with NA-91 which the RAF called "Mustang Mk IA." The USAAF naturally got a couple of those, liked them and this resulted in the P-51 (no suffix), which by the way, is mislabeled "P-51A" in the article that I just read on this website. There were about 150 P-51s, and about 50 were fitted with 2 cameras and were called the "P-51-1-NA." They kept their 20 mm cannons, by the way. They were renamed "F-6A" to reflect their photo-recon roles. Some sources even go so far as clarifying that the F-6As were later called “P-51-2-NA.” Note that the two NA-91 aircraft that had their Allisons replaced with Merlins, along with less major changes, became the XP-51B, and at first they still had the monster 20 mm cannons in the wings. Photos of them in their early stages of testing had the cannons still installed (they were WAAAY too long-barreled to miss!), and later ones had the cannons removed and the leading edge of the wings were smoothly faired over where the guns long fairings had been before. Not discussed in previous comments, though, was the fact that THIS aircraft came along at a point in history where the official NAA and USAAF name of “Apache” was given to this “family” of aircraft. The Brits naturally continued to call it “Mustang.” In mid-1942, though, both the USAAF and NAA, in a nearly unique situation, officially renamed it “Mustang,” following the RAF name. As a aside, the A-36A (NAA gave it the production name of “NA-97”). FYI , NAA gave these “NA-number” designations to all aircraft in chronological order by this simple numbering system, according to the debut of the projects, regardless of whether the aircraft was a B-25 Mitchell or an AT-6/SNJ. The first B-25 was NA-50, and the first AT-6 was NA-26, for what it’s worth. Each version of an existing NAA aircraft was given whatever was the “next number in the sequence.” So, from the NA-73x prototype, the sequence of production aircraft in the Mustang family were: NA-73, -83, -91, -97, -99, -102, -103, -104, -106, -109, -110, -111, -122, -124, -126 and -129 (for the P-51L-NT, equivalent to the P-51H [which was the NA-126, BTW] which was canceled). I posted this arcane info only to underscore the fact that the Mustang family was not composed of a series of consecutive “NA-numbers.” Interspersed in the above number sequence were I mention this because the A-36A (no prototype and the only version built – 500 total) has been misnamed “Apache” in probably 90% of all sources, be they books or websites by “experts.” The official NAA and USAAF name was always – ALWAYS – “Mustang.” Pilots in the USAAF’s 12th Air Force called it by a name that they themselves came up with: “Invader.” The pilots went so far as to petition (not sure if it was to both the USAAF and NAA or only one of them) to rename the A-36A to “Invader.” This was in vain, because Douglas Aircraft had already been given this official name and EVERY aircraft in what I call the “Mustang family” (including the F-6 line – the P-51A photo recon version was the F-6B; the P-51B/C, F-6C; the P-51D, F-6D; the P-51K, F-6K) was “Mustang” … period (or exclamation mark!), end of sentence. I’ve got a 1944 USAAF book with this info and a scan of an official 1944 US government document on naming of aircraft. Not enough “proof,” some folks say – well, the two major Mustang books clearly state this fact. Those books, “Mustang: The Story of the P-51 Fighter” by Robert Gruenhagen and “Mustang Designer: Edgar Schmued and the P-51,” by Ray Wagner. Gruenhagen was an NAA historian and Wagner was an author of a number of books on aircraft. Gruenhagen is still alive, although advanced in age and physically ill – a friend, an author in his own right, had been in contact with Bob, as he calls him, and said that his mind was still sharp as the proverbial tack. Ray Wagner has been dead for a while, AFAIK. Oh, and an online expert, Joe Baugher clearly states the fact that the A-36A, was called “Mustang.” Furthermore, having used the Boeing Archives for reference before (Boeing acquired NAA’s surviving documents that came into possession of them when North American Rockwell acquired NAA. Boeing, in turn, acquired NAR and Boeing now has the documents, photos, etc in their historical archives), I contacted Boeing’s historical archivist via email, asking if he could find references to “Apache” being given to the A-36 in anything from NAA. (BTW, I also have scans of two different pages from “Skywriter,” NAA’s employee newsletter and they both state “A-36 Mustang). The archivist replied with a clear statement when I told him that I couldn’t find ANYTHING official, that called the A-36A “Apache” and here’s his reply, copied and pasted from the email to me: "I agree, Apache was a tentative name given to the XP-51 and P-51 prior to the USAAF giving official names around the end of 1942. The name never applied to the A-36." I can see it coming – someone will say, “I’ve got proof that will trump all of your authors and sources.” The “proof”? The National Museum of the US Air Force in Dayton, OH has a beautiful A-36A named “Margie H,” on display. This museum is on my “must visit” list, BTW. The placard clearly has “North American A-36A APACHE” on it…so, there you go – it’s “Apache.” I’m going to attempt to attach it to this comment. I’ve never been a quitter, so I contacted the Curator at the NMUSAF in September, 2017. Here’s the main paragraph from the first reply from him: "Thank you for contacting the National Museum of the United States Air Force. We are circulating an update to the sign text with the correct associated nickname, "Mustang". At present, we have been unable to locate any contemporary source which designates the aircraft A-36A "Apache". Should we locate any such resource we will let certainly let you know. The nickname "Apache" will now be mentioned in the body of the text, described instead as a popular nickname. The main signage will read North American A-36A Mustang." I guess that the only thing that I need to spread more love on is that nonsense statement (I’ll be paraphrasing here) that said, “the Mustang wasn’t any kind of fighter until they put a British engine into it.” TRUE, Allison V12 as it existed in the Allison-engined Mustangs was at best, a “medium altitude” engine, and its power, and therefore airspeed and rate of climb fell off around 12-13,000 ft. What made the production Merlin Mustangs “greater,” was that the Merlin, as used in the XP-51Bs and the P-51B up through the P-51B-5-NA had two-stage, two-speed superchargers (with a liquid-cooled aftercooler) – that was a Packard Merlin Dash 3 engine. What made the Merlin Mustangs eventually even GREATER, though, was upgrading the engine to a Dash 7 which happened in the P-51B-10-NA and later Mustangs. Had the production P-51Bs and later had Dash 1 engines, they’d have had similar restrictions to the power output at higher altitudes. I say all this because there was proof that “Merlinizing” an aircraft did nothing magical to it (and NO, the Merlin got its name from a small bird of prey, not Merlin the great magician!) – the case in point was the P-40F. It had a V-1650-1 in it, and while it had a higher rate of climb for a while, it, too was NOT a high-altitude engine. Furthermore, had any of the newer Allisons be fitted into any pre-B-model P-51s – like the basic Allison in the P-63 (but obviously with a different prop reduction gearbox setup!) – they would’ve theoretically been good performers at high altitudes, too. I'm looking forward to Comments from the folks who read this. I'm "full of it" when it comes to info about Mustangs.