Both were made by North American Aviation and the planes were almost twins - the A-36 having a ground attack role while the P-51 was a fighter. Are there any noticeable differences between the two?
They were largely the same aircraft but the first line of the A-36 wiki sums it up nicely
the ground-attack/dive bomber version of the North American P-51 Mustang, from which it could be distinguished by the presence of rectangular, slatted dive brakes above and below the wings
later in the article they go on to list some more features
The contract for 500 A-36A aircraft fitted with bomb racks, dive brakes, and heavier-duty wing, was signed by Kelsey on 16 April 1942, even before the first flight of the first production P-51 in May 1942.
This website also lists the same mods as well as some other fun facts.
This website lists two other features noteably the vent window which is not mentioned elsewhere.
...and the installation of small vent windows in the windscreen side panels. In addition, two Browning M2 .50 caliber machine guns were mounted in each wing, and two more .50 cal. guns were mounted in the lower nose to fire through the propeller.
Please ... the A-36A has to have more wrong "information" on it both in print (as in books and magazines) and online in articles, photos and videos, than ANY other WWII aircraft.
First it is NOT a "derivative" or "version" of "variant" of the P-51A Mustang. It's actually the other way around. The P-51A is an offshoot of the A-36A - the last of the 500 A-36As rolled out of NAA Inglewood before the first P-51A was built.
Next, and THIS one is, for some reason, the most-repeated mistake of them all: It's called the "Apache" in books, magazines, the Internet and on videos (and, as a result, in conversations between warbird nuts in person and in social media). The National Museum of the U. S. Air Force even had their A-36A "mislabeled" on the sign in front of it, until after I contacted them. They corrected it to the only official name that it ever had, "Mustang," in September, 2018.
Here is an article that I wrote earlier this year. Right after it was published, I received even MORE documentation from the Boeing Historical Archivist that, while it was superfluous (as in, "not really needed"), it verified beyond ANY shadow of a doubt, that "Mustang" is the only official name that the A-36A ever had.
OH, and the only armament on the A-36A was the .50 Cal Browning Machine Gun - two were mounted in the nose/cheek position, and two in each wing. The 20 mm cannons were installed ONLY on the NA-91 aircraft, the first aircraft actually designated "P-51" (no suffix letters or numbers at first). THEIR official name, in the USAAC/USAAF from late 1941 until 13 July, 1942 was "Apache." After that date, anything made by NAA that even "looked like a Mustang" was called "Mustang," officially. The A-36A came along AFTER that date in July, so it was a "Mustang" from "birth."
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If I knew HOW to attach images of documents to this post, I'd gladly do it. The "Images" thing above seems to be only for images that are online, and my images are on my hard drives.
I will state that my documentation includes: scans of North American Aviation documents and newsletters and a Telegram from Dutch Kindelberger to Col Ennis, dated 13 July, '42, before the first A-36A was built; references from the most-respected "Mustang Authors" (Robert Gruenhagen and Ray Wagner); references from the US Government.
I'm actually a humble person, so if someone would tell me how to attach MY images (without first getting them online, but directly from my HDDs), I'd be most appreciative.
My only goal here is to help my fellow warbird nuts be as correct as possible when talking about our beloved warbirds - REALLY
The A-36 was a P-51A configured for ground attack with minor add-ons like the dive brakes and bomb racks (and 20mm cannon on some). Both had the Allison engine which was not suitable for high altitudes. The P-51 B and later had the RR Merlin. Lots of other detail differences, but the engine is the main one when thinking of the A-36/P-51A vs P-51B and subs.
The A-36A's main differences from the P-51A are the dive breaks, reinforced wings, nose mounted browning 50 cals, and others. The A-36 is technically a variant of the P-51. It is heavily modified but remains extremely similar. Also, contradicting @Tom Griffith, the first P-51A flew on october 26th, 1940. The A-36A apache, on the other hand, was first flown in 1942, meaning that the P-51 was actually built before. The later models of the p-51 (B,C,D,k, H...) used the merlin engine, the apache however retained the allison.