The Bell P-400 “Caribou”; Britain’s Airacobra

September 5, 2022

The Bell P-39 remains a somewhat enigmatic aircraft today, some eighty years after it first saw combat in the hands of various Allied pilots in the Second World War. Famously considered at best a mediocre aircraft by the air forces of the Western Allies, it would, in complete contrast, be ranked as one of the finest aircraft supplied under Lend Lease by Soviet pilots, who often rated it as superior to notionally much better aircraft such as the Spitfire.

And while much of this can be explained by the continuous development work conducted on the P-39 in response to feedback from the Soviet Air Force throughout the war, in fact right from the first aircraft received, the VVS pilots thought it a winner.

Which is strange, because those first models were an obscure version of the P-39 built for the Royal Air Force that the British considered an absolute dog – The Bell P-400 – initially known as the “Caribou” but more commonly known of as the Airacobra I.

So, why the difference in opinion?

First, a little history. In April 1938 Bell had flown the prototype for their new and innovative fighter aircraft – the XP-39.

This was designed with the intention of being capable of 400mph (644kp/h) and utilized a comparatively novel arrangement by having the Allison V-12 engine in a mid-mounted position behind the cockpit. This in turn allowed the aircraft to fit a 37mm cannon in the nose, a hugely powerful weapon at the time when most fighters were equipped with rifle calibre machine guns.

The novel aircraft, with its promise of both exceptional performance and heavy armament was rapidly a subject of interest for foreign buyers – something that was no doubt aided by an extremely slick advertising campaign by Bell.

The French, buying up aircraft wherever they were able (including from the Dutch!) were very keen on purchasing the aircraft, in their case the export variant designated as the Bell 14. In October 1939, with the war with Germany only a month old, they ordered two hundred P-39s, shelling out two million dollars up front on a total order value of eleven million.

The British also went for Bell’s sales pitch and in April 1940 they too ordered the Bell 14. This variant, initially named Caribou, was equipped with a 20mm cannon in the nose as at the time the 37mm gun used on the American aircraft was banned for export. However, combining the 20mm with an armament of six machine guns was a very respectable armament in 1940.


Combined with an Allison V-1710-E4 twelve-cylinder engine rated at 1150hp on take-off, the Caribou certainly looked like being a winner. Once the French orders were taken over by the British following the fall of France in June 1940, the total British order came to 675 aircraft.

The RAF was to be rather busy over the next few months what with the Battle of Britain, the Blitz, the disaster of the Greek Campaign and then the see-sawing fortunes of the battles in North Africa.

All this meant they were extremely keen to get their hands on the new American fighter as soon as possible. Because as John Dell points out in his article on the P-400, which I will link to in the description, in 1941 the RAF was still very much expecting to have to fight again over the skies of the UK. A powerfully armed and high-performance aircraft was just the ticket should the Royal Air Force find itself once again fighting for its survival as it had during August 1940.

Reports from Bell on the performance figures sounded promising, and it must have been with some excitement that, when the first three aircraft arrived in the UK in early July 1941, the pilots took the new fighter up for testing.

They were in for a bit of a shock. Instead of Bell’s promises that the aircraft was capable of close to 400mph, the service aircraft could only manage 355mph (571kp/h). Worse, performance dropped off rapidly above 13,000ft (c.4000m). For the RAF’s perceived needs for a high-altitude fighter, this was utterly inadequate.

In fact, it turned out that Bell had basically conned the British procurement committee. The high-performance statistics were all drawn from the original XP-39 prototype, which was unencumbered with weapons and other service equipment. Then, to pass the purchase committees acceptance trials before shipping to the UK, Bell had specially configured the test aircraft with special paint jobs and fairings over the gun ports, as well as reducing the aircraft weight.

This meant that the test aircraft passed muster, but the production aircraft were kind of slouches.

There were also some other significant issues with the Airacobra in terms of how it worked, or rather didn’t, with British doctrine at the time. The previous years’ experience had made the RAF appreciate the value of being able to turn around aircraft as quickly as possible once returned from combat.

This had proven a major force multiplier during the Battle of Britain and the two main British fighters, the Spitfire and the Hurricane, were capable of being rapidly rearmed and refuelled so as to be available for further sorties if need be.

The problem with the Airacobra, as the British decided to start calling it at around this point, was that the nose mounted armament and mid-mounted engine were somewhat awkward to get to and the armorers got in the way of the refuelling crew. All this greatly slowed down the aircraft’s turn around rate.


Additionally, the RAF had made great use of dispersed and basic grass landing strips as a key measure to thwart German air raids on their bases. But the Airacobra needed longer runways than contemporary British fighters, with it’s take off run being 2250 feet (686m) as compared to the Spitfire’s 1590ft (485m).

The Airacobra’s also had a host of other issues that RAF testing showed needing redressing, the sort of things that only become properly apparent when service pilots get hold of an aircraft. But even these didn’t disguise the fact that, for the RAF and the purposes they hoped to employ the Airacobra in, the aircraft was a lemon.

The lack of high-altitude performance seems to have been caused by an early decision by Bell to remove any turbocharging option from the aircraft in the development phase. Apparently, the designers thought that putting an intake for a turbo would cause more performance issues than it would solve, as they believed that the careful air streaming of the P-39 would more than compensate for this.

This decision ultimately meant that, bereft of obtaining additional power that turbo charging offered at altitude, the aircraft was never going to be able to compete at higher altitudes. This was borne out by the RAF testing, which found that it couldn’t compete against the Spitfire Mk.V above 15,000ft.

But though gravely disappointed by the Airacobra, the RAF were saddled with a large order for the aircraft that were now beginning to be delivered and needed to find a role for them.


The first operational squadron converted to the type was No.601, which traded in its Hurricanes for the Airacobra in August 1941. They flew a handful of low-level attack sorties on German positions and coastal shipping around northern France in October and again identified a few issues, such as the cannon blast disrupting the Airacobra’s compass when fired.

But the RAF was apparently not at all happy with the aircraft and by March 1942 No.601 Squadron had transitioned to Spitfire Mk.Vs – the only RAF squadron to operate the Airacobra.

So, what were the Brits to do? They had a large number of aircraft that they actually didn’t want.

Fortunately, by late 1941/early ’42 there was a whole raft of countries in dire need of fighter aircraft.

The attack on Pearl Harbor in December 1941 bought the United States into the conflict. They were desperate for fighters, and so two hundred of the British aircraft still at the factory were requisitioned by the USAAC for their use.

This is where the P-400 designation originates, and it was used by the USAAC to differentiate the type from their own P-39C-and-Ds, which were essentially the same aircraft but with different armament and fitments. Most were used for training, but a number were also dispatched to the Pacific where, still in their British camouflage, they fought during the Guadalcanal campaign.


Here they had mixed fortunes, and the British concerns over their high-altitude performance definitely is listed as issue in the histories of the campaign. In fact, pilots there had a joke that ran:

“Have you heard about the new P-400 fighter? It’s a P-40 with a Zero on its tail.”

(Military Aviation History have done a video on the P-39’s rather disastrous reputation in the South Pacific and the reasons for it, which is worth a watch.)

But though it was often badly handled by Japanese fighters, and recognized as “unable to meet the Japanese fighters on equal terms”, in the words of the USAAF’s official history, in terms of their overall performance the P-400s seem to have broadly held their own against the Japanese fighters, including the fearsome Mitsubishi A-6M Zero, once they were replaced by better high-altitude fighters and were able to be used as low level attackers. In their favour was their tough construction and heavy firepower, and down low a competent pilot could just about hold his own.

But as soon as better aircraft were available the P-400s were replaced, recognized as being not really adequate.

In addition to the aircraft available in the States, the US also used P-400s in North Africa, which were taken from the stocks of aircraft delivered to Britain but mainly left in their crates. These served alongside American P-39s with the 81st and 350th fighter groups after Operation Torch, and though the performance of these aircraft seem have been merged in the units historic records they apparently made a reasonable enough account of themselves.


What is notable about the P-400 usage with US forces in North Africa was that they were employed at lower altitudes and often served as fighter bombers, which meant that when they tangled with enemy fighters, they were not so disadvantaged by their poor high-altitude performance.

But it was with the Soviet Union that the P-39 really made its mark, and the P-400s were the first of the type that the Soviets would receive, with 212 being supplied by the British from their unwanted stocks.

And the Soviet pilots loved them, so much so that they received about half of the total run of P-39 throughout the war, over 4,700 aircraft, and the vast majority of the later P-63 Kingcobra.

So why did Soviet opinion differ so much from the RAF’s?

Well, like their usage in Africa, the P-400s were operating in a more fluid and less-controlled environment than RAF operations employed over the UK. This meant that the P-400s generally operated at lower altitudes as they were aiming to attack German tactical aircraft, especially bombers. These operated at lower altitudes so as to identify and attack their targets, and their escorting fighters had too generally do likewise.

This not just negated the P-400s altitude disadvantage, it put them firmly in the zone they operated at peak efficiency, especially against contemporary German aircraft.

In fact, the RAF had already discovered this. When they conducted their fly offs between the P-400 and the Spitfire, they had also included a captured Bf-109E in the mix. This had found that the P-400 could out turn and out dive the German fighter at altitudes below 15,000ft and was considered a worthy match for the Messerschmitt.

Considering that the Soviets had lost an estimated 4,000 aircraft in the first week of the German attack on them and that many of their remaining fighters in the latter half of 1941 were inferior to those of the Germans, their enthusiasm for the P-400 – modern aircraft capable of taking on the Bf-109 – also makes sense.


And there is another factor that often gets overlooked in the Soviets requests for as many P-39s as they could get, and again it has to do with their first impressions of the aircraft they received as aid in the dark days of the latter half of 1941 and the first half of 1942.

As Valery Romanenko points out in his book “Airacobras Enter Combat”, the initial shipments of aircraft that the British sent to the Soviets were often worn-out Hurricanes and P-40C Tomahawks that had seen hard use in RAF service already.

But the P-400s were brand new, often still in their crates.

Even better, the British had already identified many of the issues that the Airacobra had in service and so either informed the Soviets of the problem or rectified it in the stored aircraft prior to shipping.

With all those factors in mind, it is hardly a surprise that the Soviets rated the P-400 so highly, in notably sharp contrast to the British, who had a completely different set of expectations that, when they proved false, pretty much gave up on the aircraft. And this popularity no doubt played a role in the Soviet’s continued usage of the P-39 right up to the end of the war.

As many of the top Soviet aces scored a lot of their kills, in many cases the majority of them, in P-39s, that would indicate that the P-400 had a greater role and influence on the aerial war of the Second World War than is generally appreciated.


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