1937-45: Doubling down on the 2-engined 'day fighters'

don4331 said:

They did do the math - minimum distance from Germany to UK is ~250 miles one way, more if you avoid Dutch airspace; even Bf.110 isn't able to make it to much of England on internal fuel.

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Perhaps the failings were in the recent history, with RAF planners forgotten that Germans - and their aircraft - were just across the channel back in 1914-1918. Counting on the best case scenario - Germans will leave Belgium alone just because it is convenient to the British defenses - is/was bad planning.
And this was with ~1/2 of the German army deployed against a numerous and aggressive opponent back in the ww1. Now - in the 1930s - there is no such threat to the German eastern borders, so the Germans can deploy even greater % of their forces to the West.
Add the stance of British government until too late that there will be no continental deployment, and math, indeed, does not favor the notion that Germany will not be occupying even greater chunk of the real estate between Rhine and the Channel.

don4331 said:

Defiant was reasonably successful versus bombers when single engine fighters weren't around. It also did OK against Bf.109s until they realized it could shoot back and went to head on attacks.

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What were the kill numbers vs. losses for the Defiants in 1940? Like real kills, not claims?

don4331 said:

They did do the math - minimum distance from Germany to UK is ~250 miles one way, more if you avoid Dutch airspace; even Bf.110 isn't able to make it to much of England on internal fuel. Defiant was reasonably successful versus bombers when single engine fighters weren't around. It also did OK against Bf.109s until they realized it could shoot back and went to head on attacks.

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The Defiant was a replacement for the Hawker Demon, which is where the basic math calculations came off the rails.
Hawker introduces the Hart bomber. This can handily outrun the current RAF fighters (Siskin and Bulldog) so to get something that can intercept the Hart (or foreign equivalents)
And Hawker offers the Demon in the line of 'it takes a Hart to catch A Hart". Now the Fury could catch the Hart but in the production quantities being ordered the Fury was more expensive per plane despite being smaller (less raw material and labor) and the Demon was enshrined as a needed type of aircraft and a successor to the B.2F, in part because it was cheaper, not better.
Using the same engine a single seat fighter could have been armed with 4 guns (should such a gun have been available) for less weight than the Demon. The Demon with a single Lewis gun in the back, even with a powered mount (they were finding out that muscle powered guns didn't work well even on 180mlh aircraft) was hardly an outstanding bomber destroyer. The Vickers guns out the front were little better than 1918.
The Brownings were much better and perhaps the idea of 4 of them (8 times the fire power of the Lewis) swayed the PTB (powers that be) but the same 4 guns in a single seat fighter also offered 4 times the firepower of Siskin/Bulldog/Fury and the Hurricane was offering about 8 times the firepower of the biplanes (or twice the bullets per second of the Defiant.)

Somebody got in their heads that the speed of modern aircraft made it too hard to aim (without much being done in the way of testing?) and the two seat fighter with power turret was the answer.

don4331 said:

It also did OK against Bf.109s until they realized it could shoot back and went to head on attacks.

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Which took all of a few weeks, a rather poor return on a 5 year program (requirement issued in 1935, Plane has to be taken off day light duty in the summer of 1940)

don4331 said:

There is also pilot training:
For single engine, you have basic training, advanced training and then type training
For twins: you have basic training, advanced training, basic twin, advanced twin and finally type training. And to muddy the water, often twin includes night and instrument training.The twin trainers are expensive planes (they burn 2X the fuel) and the pilots need to be proficient in flying* twin with only one engine running.

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You are giving a little too much credit to the British training program of the time. Also as regards the Whirlwind, the pilots only got the standard "night and instrument training" which was pretty poor. (so was everybody else in 1940). Night fighting in 1939-40 without radar basically meant that the pilot had taken off and landed a few times at night and had used either 1-2 flares to illuminate the landing field or had landed at a field using an illuminated runway, ground flares or lamps. Not much advanced from WW I.
I think the Whirlwind squadron/s (2) had 1-2 Ansons attached for conversion to twins for the pilots. The Pilots did NOT show up with a 'twin' rating. One can well imagine the change from the Anson to the Whirlwind.
I think that some of the early Beaufighter squadrons did the samething, although some also had a few Blenheims to also help the pilots transition.

By the way, the likely result of trying to fly an early British twin on one engine was that you simply crashed a some point after the 1st engine stopped.
Took a while for the British to admit in public that fully feathering propellers were NOT the work of devil worshipers.
Some British twins continued to fly (badly) with one one prop windmilling. Some allowed the pilot to select max course pitch setting and some of the lucky ones were fitted with a brake to stop the rotation of the dead engine as the airstream fought the non-feathered propeller.
A lot of the early British bombers (and some of the light twin trainers?) had negative rate of climb on one engine. How far you could fly depended on how high you were when the engine crapped out.

tomo pauk said:

What were the kill numbers vs. losses for the Defiants in 1940? Like real kills, not claims?

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I have always wondered if they lost more Defiants on nightfigher missions due to crashes than they came close to shooting down?
The Defiant saga may have been heavily into the negative column, both day and night.

Shortround6 said:

Somebody got in their heads that the speed of modern aircraft made it too hard to aim (without much being done in the way of testing?) and the two seat fighter with power turret was the answer.

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Never let facts interfere with a good theory.

The defence of the Defiant can be found in The Defiant File by Alex Brew.

Turret idea tested on Sidestrand, gunner score went from 15 to 85%. helps explain why people believed in the self defending bomber.

Specification F.9/35 two seat day and night turret fighter/bomber destroyer performance approaching F.10/35 single seat fighters, 298 mph, 5.5 minutes to 15,000 feet, able to carry 20 pound bombs for army co-operation. Boulton Paul licensed a French turret design. Production and development delayed as Boulton Paul became involved in the Roc. Specification ignored the lesson of the Bristol fighter of give the pilot some guns.

264 squadron began night flying practice in February 1940, wide track undercarriage, good pilot view, they did dive bombing and strafing exercises and also came up with defensive circles against enemy single seat fighters.

29 May 1940 Defiants claimed something like 37 kills, mostly bombers. Overall a mixed result over Dunkirk, needed to fly defensive against Bf109 otherwise largely offensive. 19 July it was 141 squadron, did not use defensive circle, were alone when they ran into part of III/JG52, 6 out of 9 Defiants lost. 24 August claims of 8 bombers and 1 Bf109 for 4 Defiants. 26 August took on the escort while the Hurricanes hit the bombers, 6 bombers and 1 Bf109 for 3 Defiants. All up to late August day fighting claims for 88 German aircraft destroyed for 32 Defiants lost.

Top scoring night fighter 1940/41, which was not a lot, and plenty of accidents, turned more contacts into destroyed, still as good as Havoc and Beaufighter 1941/42.

Drawings for AI mark IV done November 1940, details of the radar received the "previous autumn", Defiant fitted with radar delivered to Special Duties flight 23 April 1941

The Defiant I had 104 gallons of fuel, range 465 miles at 260 mph at 260 mph, the mark II had an extra 55 gallons in wing tanks

Target Tow prototype 31 January 1942, ASR duties April, Radar Counter Measures May, day raids, making the force seem bigger than it was.

Remembering the following is not the full price, but the money paid to the manufacturers

Whirlwind, first 24 were 15,370 pounds each including 2,256 for jigs and tools, last 90 were 11,719 pounds each including 2,257 pounds for jigs and tools.
Beaufighter 16,430 pounds for first 104, next 45 were 11,107 pounds. Mark II with Merlin 10,550 pounds, Mark VI 9,650 pounds.
Defiant 14,000 pounds each for first 87 including 3,355 pounds jigs and tools. Next 75 were 6,828 pounds, next 125 were 6,468 pounds, next 771 aircraft 6,250 pounds.
Hurricanes first 298 were 4,750 pounds, reduced to 4,000 pounds for next 220.

TM06 said:

Japan is an interesting case because they have two of the most compelling reasons for a twin-engined fighter: long range requirements and a desperate need of Interceptors. The Ki-45 and J1N were decent but not nearly enough to pose a genuine threat to the vast swathes of American heavy bombers.
Fortunately, there were two aircraft in particular that could pick up that slack - the Ki-46-III and the Ki-96. The Ki-46-III was a good bit faster than the Ki-96 (390 mph to the Ki-96's 370 mph) but the Ki-96 climbed much better (16.6 m/s to the Ki-46-III's 7.5 m/s) and both of them wield a quite heavy armament package of 2 x 20 mm cannons + 1 x 37 mm cannon (obliquely mounted Schräge Musik style for the Ki-46).
The biggest draw of these two in my opinion is that they don't use the volatile Homare or Ha-43, they use the highly reliable and widely produced Kinsei (specifically the Kinsei 62 making 1,500 hp).
Had the requirement for the Ki-96 not randomly shift back to a two-seat fighter, it absolutely would have entered mass production and would likely have been a strong weapon against the bomber horde.

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Against P-51D or P-47N? Unlikely that they would have been a strong interceptor if they had to a.) penetrate, and b.) evade pursuit. The FW 190D and 109K with far superior performance against much slower bombers than the B-29 did not do well, simply because they weren't 'unmolested'.

Shortround6 said:

Unfortunately the availability of the Gladiator and Blenheim meant that the better planes could be kept at home while the Gladiator and Blenheim went off to defend the empire.
The Italians and Japanese did not get the memo that they were supposed use old obsolete aircraft in 1940-41-42 just to keep thing fair.

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Actually the Italians did play fair using the Fiat CR 42 for the entire time they were in the war.

Shortround6 said:

By the way, the likely result of trying to fly an early British twin on one engine was that you simply crashed a some point after the 1st engine stopped.
Took a while for the British to admit in public that fully feathering propellers were NOT the work of devil worshipers.
Some British twins continued to fly (badly) with one one prop windmilling. Some allowed the pilot to select max course pitch setting and some of the lucky ones were fitted with a brake to stop the rotation of the dead engine as the airstream fought the non-feathered propeller.
A lot of the early British bombers (and some of the light twin trainers?) had negative rate of climb on one engine. How far you could fly depended on how high you were when the engine crapped out.

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One of the surprising facts is that large portion of American bombers losses were due to unsuccessful attempts to feather propellers.
Even more surprising to find that half the USAAF heavy bomber loses were due to engine damage, despite the many sources that state that American radial engines were highly resistant to damage
From Rand Paper RM-402
"Aircraft Vulnerability In World War II
II Damage Causing Loss of Aircraft"
European Theater of Operation"

"….that of 100 lost bombers , fifty went down with engine mechanical trouble only, thirty-six with fires only, and fifteen with explosions only."

"A study of 282 lost bombers showed that in 188 cases on 137 aircraft where the attempt was made to feather a propeller, the attempt failed in eighty cases, or 43 per cent , with the inability to feather traceable in nearly every case to leaks in the oil supply. Of these feathering attempts, fifty per cent with the B-17 failed and twenty-five per cent with the B-24. The B-17s included in this sample had no auxiliary feathering system, but many of the B-24s did have a reserve supply of oil for feathering."

One of the major blunders made by the AAF was deleting the back up feathering systems on B-17s. I believe it was reinstated before the end of the war

drgondog said:

Against P-51D or P-47N? Unlikely that they would have been a strong interceptor if they had to a.) penetrate, and b.) evade pursuit. The FW 190D and 109K with far superior performance against much slower bombers than the B-29 did not do well, simply because they weren't 'unmolested'.

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That's an extremely unfair comparison. The Ki-96 and Ki-46-III ideally would have been in production long before the B-29 came into play, 1942 being the most reasonable assuming everything went well.
A more fair comparison would be the Ki-96 and Ki-46-III against the Bf 110 F/G and Me 210 A or P-38G - in which case they stack up very favourably.

Shortround6 said:

I have always wondered if they lost more Defiants on nightfigher missions due to crashes than they came close to shooting down?
The Defiant saga may have been heavily into the negative column, both day and night.

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Which of the two is the worst use of limited Merlin production, 1k Defiants or 2k Battles? Imagine swapping those three thousand Merlins into Spitfires and Hurricanes.

Geoffrey Sinclair said:

The defence of the Defiant can be found in The Defiant File by Alex Brew.

Turret idea tested on Sidestrand, gunner score went from 15 to 85%. helps explain why people believed in the self defending bomber.

Specification F.9/35 two seat day and night turret fighter/bomber destroyer performance approaching F.10/35 single seat fighters, 298 mph, 5.5 minutes to 15,000 feet, able to carry 20 pound bombs for army co-operation. Boulton Paul licensed a French turret design. Production and development delayed as Boulton Paul became involved in the Roc. Specification ignored the lesson of the Bristol fighter of give the pilot some guns.

264 squadron began night flying practice in February 1940, wide track undercarriage, good pilot view, they did dive bombing and strafing exercises and also came up with defensive circles against enemy single seat fighters.

29 May 1940 Defiants claimed something like 37 kills, mostly bombers. Overall a mixed result over Dunkirk, needed to fly defensive against Bf109 otherwise largely offensive. 19 July it was 141 squadron, did not use defensive circle, were alone when they ran into part of III/JG52, 6 out of 9 Defiants lost. 24 August claims of 8 bombers and 1 Bf109 for 4 Defiants. 26 August took on the escort while the Hurricanes hit the bombers, 6 bombers and 1 Bf109 for 3 Defiants. All up to late August day fighting claims for 88 German aircraft destroyed for 32 Defiants lost.

Top scoring night fighter 1940/41, which was not a lot, and plenty of accidents, turned more contacts into destroyed, still as good as Havoc and Beaufighter 1941/42.

Drawings for AI mark IV done November 1940, details of the radar received the "previous autumn", Defiant fitted with radar delivered to Special Duties flight 23 April 1941

The Defiant I had 104 gallons of fuel, range 465 miles at 260 mph at 260 mph, the mark II had an extra 55 gallons in wing tanks

Target Tow prototype 31 January 1942, ASR duties April, Radar Counter Measures May, day raids, making the force seem bigger than it was.

Remembering the following is not the full price, but the money paid to the manufacturers

Whirlwind, first 24 were 15,370 pounds each including 2,256 for jigs and tools, last 90 were 11,719 pounds each including 2,257 pounds for jigs and tools.
Beaufighter 16,430 pounds for first 104, next 45 were 11,107 pounds. Mark II with Merlin 10,550 pounds, Mark VI 9,650 pounds.
Defiant 14,000 pounds each for first 87 including 3,355 pounds jigs and tools. Next 75 were 6,828 pounds, next 125 were 6,468 pounds, next 771 aircraft 6,250 pounds.
Hurricanes first 298 were 4,750 pounds, reduced to 4,000 pounds for next 220.

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The Defiant was intended to use the turret guns fixed in a forward firing position over the propellor from the gun firing button fitted for the pilot and needing a sight for him for that 'no deflection' firing purpose. That idea was somehow lost in the development and service trials yet the button was in place on all Defiants.

So the Defiants were intended to have forward firing guns at no extra weight.

Admiral Beez said:

Which of the two is the worst use of limited Merlin production, 1k Defiants or 2k Battles? Imagine swapping those three thousand Merlins into Spitfires and Hurricanes.

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There maybe something to be said for the stopping the Defiant production.

The Battle production gets a lot iffier. It is not quite a zero sum game.
Battle production started in late 1936/very early 1937. First operational squadron (No 63) got their first planes in May 1937.
Early Battle production used Merlin Is, Prototypes had used a Merlin C and a Merlin G.
There were a lot of problems with the Merlin I and after 136 planes had been built they switched production to the Merlin II.
This did affect Hurricane production but in the end the Hurricanes got the better engines.

There is also the problem that Hurricanes and Spitfires could not do the job the Battle was supposed to do. In Sept 1939 15 bomber squadrons were equipped with Battles.
Yes the Battle was not a good bomber in Sept 1939 but if you don't build them then you have 15 bomber squadrons flying around with Hawker Hart biplanes (or slightly newer versions)
using overhauled engines. Maybe Vickers could build a few more Wellesley bombers? Or maybe Fairey could have figured out how to stick a Pegasus in the nose of the Battle instead of the Merlin for a truly slow bomber.

Battle bomber operations in France make for depressing reading, but a lot of that has to do with poor doctrine, tactics and training (they were not training enough for the job they were called upon to do). But 10 Hurricane squadrons in April/May of 1940 were not going to slow down the German army much better than the Battle squadrons. Attacking the Bridges with .303 guns wasn't going to work.

The Defiant was around 1 1/2 to 2 years behind the Battle. Only 3 production aircraft had been delivered as of Sept 1st 1939. Impact on Hurricane or Spitfire deliveries in 1939 would be minimal. About 45 had been delivered as of Jan 1940 and here is where things get a bit murky. The first order for 87 planes had been placed back in April of 1937, the 2nd order for 202 planes had been placed in Feb 1938, April or June of 1938 saw another 161 ordered. This was about 1 year before the 1st production plane flew at the factory. But in Dec 1939 another 150 planes were ordered (total 513) but by early 1940 another 50 planes showed up on the books and then another 280 were ordered under a manufacturing plan that was re-arranged in summer of 1940. Now how much "stuff" was pre-ordered (Brakes, landing gear, hydraulic parts, forgings, etc) I don't know but they decided to keep manufacturing the things for economic reasons. They also ordered the MK II version with Merlin XX engines in the summer of 1940.
Now whatever they may have thought in 1935-37 about the ability of fighter pilots to aim the guns from a 300mph fighter they had experience in 1939 to show it was not the problem they had imagined. Maybe they didn't know how well the Defiant turret would have worked or not worked at the end of 1939 but they knew the basic reason was false (that the turret was one of the main solutions). A more ruthless alteration of Defiant production might have saved several hundred Merlin IIIs and all of the Merlin XXs that went to the later Defiant production. How much scrap from early lead parts that would cost I don't know. You also have to come up with hundreds of target tugs

How much actual fighting was done until May, 1940?
Back on the original subject, the obvious twin fighter is the Bf.109Z.
I think you may do the same thing with the Spitfire. (Hurricane's wing doesn't allow for easy twinning; not completely sure about P-40).

don4331 said:

Back on the original subject, the obvious twin fighter is the Bf.109Z.
You ask Willi for a couple Bf.109B fuselages with engines and a set of wings. You return the horizontal stabilizer and elevators. Then build the connecting center wing & horizontal stabilizer. We need to add engine controls to the cockpit (engine gauges will need to be in cowl) You get a fighter with 7 to 9- MG17s (outer wings one each, one in center wing, 2 in each cowl (maybe 2 firing through engine) <alternately replace wing guns with 20mm cannons>). Navigator/longer range radios can be in 2nd fuselage.With the concept proven, move on to the Me.109E fuselage/DB601 engines. Beef up center section to allow 900 liter drop tank; alternatively 300 liter drop tanks under fuselage and 500kg bomb on centerline for the Me.109Z-2/B.The F/G based twin would be competitive with s/e fighters until the SuperProps/jets arrive.I think you may do the same thing with the Spitfire. (Hurricane's wing doesn't allow for easy twinning; not completely sure about P-40).

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100% agree on twin 109 and Spitfire.
Here is how a twin Spitfire might've looked, from some time ago:



Wings are clipped.
Main shortcoming is that Spitfire took a long time to make, and was expensive as-is (here the 109 shines, on the contrary). But then again, we remove a big chunk of the wing per aircraft, that was both expensive and time-consuming to make due to the forest of tiny pieces making up the ribs (plus the opportunity to make the central wing in a simper fashion?). Just two main U/C legs & tires, but stronger than on the regular Spitfires. Wing area of some 350 sq ft?

don4331 said:

How much actual fighting was done until May, 1940?

Yes, the war started in September,'39 and there were some clashes on the Western Front, but by middle of October, Poland as lost and the Phoney war was really more of a cold war both on the ground and in the air (On the water, things were pretty hot).
So, it is June/'40 before the mistakes are obvious and then you have all the inertia in the pipeline.
​

Back on the original subject, the obvious twin fighter is the Bf.109Z.

You ask Willi for a couple Bf.109B fuselages with engines and a set of wings. You return the horizontal stabilizer and elevators. Then build the connecting center wing & horizontal stabilizer. We need to add engine controls to the cockpit (engine gauges will need to be in cowl) You get a fighter with 7 to 9- MG17s (outer wings one each, one in center wing, 2 in each cowl (maybe 2 firing through engine) <alternately replace wing guns with 20mm cannons>). Navigator/longer range radios can be in 2nd fuselage.
With the concept proven, move on to the Me.109E fuselage/DB601 engines. Beef up center section to allow 900 liter drop tank; alternatively 300 liter drop tanks under fuselage and 500kg bomb on centerline for the Me.109Z-2/B.
The F/G based twin would be competitive with s/e fighters until the SuperProps/jets arrive.
​

I think you may do the same thing with the Spitfire. (Hurricane's wing doesn't allow for easy twinning; not completely sure about P-40).

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The Bf109Z project was based on the Bf109F...

GrauGeist said:

The Bf109Z project was based on the Bf109F...

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This reminds me of the contradictions of German nomenclature. Of course the Bf 109Z follows the F, but what's with the Panther Ausf. A following the Ausf. D?

Admiral Beez said:

This reminds me of the contradictions of German nomenclature. Of course the Bf 109Z follows the F, but what's with the Panther Ausf. A following the Ausf. D?

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Well to confuse you even further when the German Army contracted MAN in May 1942 to produce the first pre-production versions it was designated Ausf A. In Jan 1943 it was redesignated the Aust D, the first 4production tanks being produced between Nov 1942 & Jan 1943.

I've no idea why that change of designation was made. And then what happened to B to F before Ausf G arrived?

Shortround6 said:

There is also the problem that Hurricanes and Spitfires could not do the job the Battle was supposed to do. In Sept 1939 15 bomber squadrons were equipped with Battles.
Yes the Battle was not a good bomber in Sept 1939 but if you don't build them then you have 15 bomber squadrons flying around with Hawker Hart biplanes (or slightly newer versions)
using overhauled engines. Maybe Vickers could build a few more Wellesley bombers? Or maybe Fairey could have figured out how to stick a Pegasus in the nose of the Battle instead of the Merlin for a truly slow bomber.

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As we've learned, not just in this thread, a number of Blenheim fighter squadrons were formed. So let's have the Merlins, saved from the Battle production being dialed-down, installed on our brave new 2-engined fighters, while these Blenheims never became fighters.
AM can try and get the Blenheims with the increased payload, so the bombload per squadron is more favorable (as it was the case with Finnins Blenheims, including the Mk.Is). Also, less pilots is needed.

Even better if AM has, perhaps, Pegasus-powered Beaufort on order at Bristol, as a back-up should the newly-fanged Taurus is giving problems.
Better still, make more of the Hampdens.
A Pegasus-powered Henley might also save a lot of Merlins.
Granted, each of the 3 Pegasus options excludes other two.

Shortround6 said:

Battle bomber operations in France make for depressing reading, but a lot of that has to do with poor doctrine, tactics and training (they were not training enough for the job they were called upon to do). But 10 Hurricane squadrons in April/May of 1940 were not going to slow down the German army much better than the Battle squadrons. Attacking the Bridges with .303 guns wasn't going to work.

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If these 10 Hurricane squadrons are retained in the UK, then indeed they will do nothing to prevent the disasters of May 1940.
If they are based in France, then they might deal greater losses to the LW, and hamper the progress of Heer that many times use LW bombers to clear up the obstacles, and obviously fighters to blunt the Allied air attacks.
Bridges will still be attacked, by Blenheims and French bombers.

Shortround6 said:

The Defiant was around 1 1/2 to 2 years behind the Battle. Only 3 production aircraft had been delivered as of Sept 1st 1939. Impact on Hurricane or Spitfire deliveries in 1939 would be minimal.

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If RAF decides that the best bomber destroyer is a high-performance fighter armed with heavy front-firing battery (that is significantly heavier than the 8 .303 battery for the new 1-engined fighters), the turret fighter idea is no longer there by 1935/36.
That means no Defiant.
That in return means that BP makes something else, perhaps a 2-engined (2x Merlin) fighter armed with 12 .303s ( 4 x 20mm when available), or Hurricanes, or Spitfires.

This also means no Whirlwind, so there is an even easier path for RR (no small series engine, that requires both the resources of the design office, as well as it's own production line = more Merlins), and we have Westland making something else from late 1939 on.

Admiral Beez said:

This reminds me of the contradictions of German nomenclature. Of course the Bf 109Z follows the F, but what's with the Panther Ausf. A following the Ausf. D?

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The "Z" suffix denotes "Zwilling" of course, just as the "T" stood for "Träger".

I don't think that Germany intended to keep the Bf109 in production long enough to use up most of the alphabet.

What is interesting, though, is the Zwilling project based on the Me309 was known as a Me609 and not a Me309Z.

tomo pauk said:

If RAF decides that the best bomber destroyer is a high-performance fighter armed with heavy front-firing battery (that is significantly heavier than the 8 .303 battery for the new 1-engined fighters)

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Before the advent of the 20mm cannon what would that be? Perhaps ten .303 mgs? There is the 0.55 in. used in the Boys anti-tank rifle.

It can take several years (sometimes as many as 6) to go from concept to working aircraft gun. Even converting a ground gun can take several years depending on the test standards.
A lot of WW I machine guns jammed a lot, a major reason they were located where the pilot/crew could get to them.

And once you have a gun you have to decide on what kind of ammo, that means bullets/shells. Using .5/.50cal/.55/13mm bullets means with solid bullets you get more penetration than 7.5-8mm bullets but the holes don't get that much bigger. Using exploding ammo means much bigger holes. Much more expensive ammo per round. It also means a lot more testing and judgements about legality.
By 1938 the British were already investing in the 20mm Hispano and nothing else was going to get into production and into service squadrons any quicker.
Inventors and companies may have had hopes but reality is harsh.

Getting .50 cal Brownings from the US was sort of possible. As was the Belgian 13.2mm but here actually reality intrudes.

Britain needed guns by the 10s of thousands per year and by 1942 that many per month. The US only achieved the number of .50 cal guns it did by tooling numerous factories for it.
Total production in 1940 was under 8000 guns. Britain's chances of getting any sort of heavy machine gun into sizeable production in 1939-40 was very slim.
And Britain in 1939-40 was very close to a zero sum game. For ANYTHING you want to make, aircraft gun, tank gun, naval weapon, pistol, there is something you cannot make.
Which is one reason that the British wound up with some of the goofy crap like the "Smith gun" and the Northover Projector.
Enthusiasm overcame common sense.