The Twin 40mm Mustang; AM106

June 2, 2023

It is well remembered that the legendary P-51 originated out of a…dare I say boast?…made by “Dutch” Kindelberger, President of North American Aviation. Asked by the British if they could build the Curtiss P-40 under licence for the Air Forces of the Commonwealth, Kindelberger famously said that North American could design a far superior fighter in the time it took to get the P-40 into production. From such things legends are born, and the foundation was laid for the P-51 Mustang.


But the aircraft didn’t leap into its role as a formidable fighter straight off the bat. I’ll have to do a dedicated piece one day on the development of the Mustang, but the long and the short of it is that it started off in a very different role. When the first Mustang’s, the Mk.Is, were delivered to the British starting in October 1941, the RAF assessors soon determined that they had a very formidable aircraft on their hands.

These were armed with four .50 and four .30-calibre machine guns and had blazingly good performance…at low level. In common with the early P-40’s, the American Allison engines supplied to the British suffered power drop off above 15,000 feet (c.4,570m).

So, the RAF determined that the best use for their new aircraft was as a low-level reconnaissance platform and ground pounder, roles that the first Mustangs excelled in.

And at about the same time the RAF had also began conducting combat operations in North Africa with a new aircraft weapon expressly intended to rip open tanks; the Vickers S 40mm gun. This pod-mounted weapon was fitted under the wings of Hawker Hurricane’s to form the IID variant and initial combat experience showed that it could be rather effective. Indeed, No.6 Squadron, the first to be equipped with the Hurricane IID, rejoiced under the nickname of “The Flying Can Openers”.


But though the Hurricane IIDs were certainly dramatic, they were also rather vulnerable. The weight of the gun pods, additional armour and air filter for North African operations significantly degraded the aircraft’s performance, meaning that the -IIDs could only operate safely when not facing German fighters or significant air defences. So, it was entirely logical that the British have a look at maybe fitting their new, high speed ground attacker with the S-gun.


AM106 was one of several of these first Mustang’s that was allocated for stores testing and trialed zero length rocket launchers, drop tanks and ultimately two 40mm gun pods. And to be honest, I haven’t been able to find anything on the test reports on the aircraft, so if anyone knows anything feel free to drop me a line.

Because all that remains, as far as I can find, are some intriguing photographs of AM106, which obviously conducted flying and firing trials with the 40mm Vickers, but this weapon was never adopted by the RAF Mustang Squadrons.


To be honest I suspect that the British testers concluded that the Mustang Mk.I was formidable enough as a ground attack aircraft with rockets and bombs, without the encumbrance of the bulky gun pods. Plus, though the Vickers S would see use in small numbers throughout the war, I further suspect that the RAF really didn’t see much point in making heavy future commitments to it as other ground attack weapons offered far greater flexibility for development.

But it did make for one cool looking aircraft.



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