The XA-38 Grizzly

[tomo pauk]

I'm a retired artilleryman by trade, so I know a bit about the actual effect of cannon shells. History actually tells us that aircraft cannon used against ground or naval targets were largely inneffective, and the 75mm is particularly puny. An aircraft can't carry the weight of a cannon, or absorb the recoil, to fire a shell large enough to really be effective.

If you want examples of what it takes and the limitations of cannon, read anything about employing submarine guns against naval targets. It sometimes took hundreds of hits by cannon far larger than any aircraft could carry to sink a ship. U.S. subs with 3 or 4 inch naval cannon were replaced by 5 inch cannon late in the war, but the whole idea of naval gunnery for subs was scrapped right after the war, due largely to ineffectivness.

Likewise, as good as 75mm B-25s were supposed to have been, why were they scrapped even before the war was over, and replaced with additional .50 Cal MGs?

The idea of aircraft cannon sounds cool, but it never really panned out. Otherwise, we'd have seen many more cannon armed aircraft.

What the man said.

The USAF and USN units using the B-25/PBJ in anti-shipping duties were far better off with skip-bombing and mast-height bombing. Even a small 250 lb bomb will do much more damage to any ship than a 14 lb shell the 75mm was firing. People might want to check the accounts of the Battle of Bismarck Sea (Wikipedia article), to see how devastating such bombing tactics were against destroyers and merchant shipping. The skip-bombers were supported by fighters, heavy bombers and RAAFs torpedo bombers, the most of the sinks was accomplished by skip-bombers. The 75mm used against anything bigger than destroyer escort would just scratch the paint of the intended target, while the B-26 with 8-12 HMGs in the nose can also suppress most of the target's AAA gunners much better than it would be possible with only 4 HMGs.
75mm was also ineffective against the targets in revetments, and parachute bombs were excellent to take out the such protected targets. The RAF quickly 'forgot' the excellent, high RoF Molins gun, and issued rockets to it's Mosquitoes.

 

[GregP]

I see in that clip that several Mosquitoes salvoed off all 8 rockets and hit nothing but water. I doubt that a decent pilot would shoot 8 cannon rounds and miss by that much.

I'm not very impressed with WWII rockets in all the combat footage I have seen. Yes, they got some hits but, if you add up everything you see, the hit percentage is very low. I'd estimate somewhere around 20 - 30 % from the footage I've seen. Altogether a relative waste without 4 or more aircraft all shooting at the same target for a decent chance of a hit or hits. When they DO hit, I see it is usually several from within the salvo, and that, at least, is a good thing.

Let's not argue about it. I don't like WWII rockets and you do and that's fine. In the end, they did what they did, and neither of our opinions will change the events, so it's down to a matter of preferences. We all have our own.

I'll take a cannon-armed attack platform any day over rackets, and if I could choose any of them, I;d probably take a plane with for or more 20 or 30 mm cannons. If you DO get hits, you'll get damage and mostly not minor damage unless you hit the armor of an armored ship or the dedicated armor on a tank. If so, there is probably a direction from which you can destroy almost any tank. For the armored ship, you'd probably need bombs or torpedoes.

For almost anything else, a quartet or more of decent cannons are a tough prospect. If I could choose, I like the Soviet cannons. Particularly the NS 23 and 37 mm varieties.

Everyone else probably also has their favorite suite of aircraft armament.

 

[yulzari]

The 6 pounder 57mm Molins autocannon used by the Mosquito was made in a 75mm version as the standard Royal Ordnance Factory tank gun using the same ammunition as the US 75mm so a Mosquito could have had a 75mm HE with a higher ROF than the XA38 (or the XA38 could have been fitted with the Molins auto loader). Tank experience suggested that 75mm is the minimum effective size for HE fire support.

Regarding comparative effectiveness: Hurricane MkIVs in Burma 1944/5 found the twin 40mm S guns with HE rounds to be more effective than the x8 60lb unguided rockets. Not in terms of energy but in accuracy. With the guns you could hit your target. With the rockets you hoped that one might get near your target out of all 8.

However, the exponential increase in weight of guns, as the calibre increases, puts a 75mm gun near the practical limit. Rockets allow bigger, if less accurate, bangs as do bombs. For light craft and surfaced submarines the accuracy of guns may well be the optimum choice. For bigger ones rockets, and anything over destroyer size needs bombs, torpedos, Highballs or mines to deal with them.

I would be interested to know what were the targets that the XA38 was designed to engage?

Personally I feel that a pair of 40mm cannon with a good ammunition supply and thin case HE warheads in a small airframe would be the optimum for close air support. For naval targets the 75mm gun but the advantage of rockets, bombs, torpedos and drop tanks is the flexibility to tailor your warload to a variety of situations; from S boats to shipyards.

 

[Aozora]

Much as there might be arguments about the effectiveness of the 75mm the fact is that a 14lb high velocity high explosive shell is still an effective weapon against many targets; unlike the unguided rockets that had steeply curved trajectories, a flat trajectory 75mm shell could be aimed to hit a target from a distance. The opinions of a veteran who actually used the 75mm armed B-25 in combat should count for something (Jerry Scutts B-25 Mitchell at War, pages 48-51):

The fact is the 75mm was more effective than muskeg and tomo are alluding to;

As far as I'm concerned the cannon did a real fine job the way we used it....It was very effective against shipping....We found that the cannon was more precise against some targets than bombing...

the limiting factor, and the reason the 75mm was eventually dropped from the B-25s, was the damage done to the airframe, something which the purpose-built installation of the XA-38 would have helped alleviate.

Attached is a really interesting article from an olde but goode November 1978 Airpower (16 MB):

View attachment Beech XA-38 'Grizzly'.pdf

BTW, the Tsetse Mosquitoes stayed operational until VE-Day - they were not rejected.

 

[muskeg13]

from Life and Times of the 341st Bomb Group
www.usaaf-in-cbi/341st_web/aircraft/b25h.htm

"Combat sorties confirmed that the cannon armed B-25H offered no particular advantage over specially adapted strafers armed exclusively with machineguns. At this stage in the war, targets specifically suited for cannon attack were few and far between, and many targets that were vulnerable to cannon were also vulnerable to a battery of 0.50-inch machineguns or to bombs. Consequently, the use of heavy cannon was generally abandoned in the South-West Pacific by August of 1944."

 

[GrauGeist]

"Combat sorties confirmed that the cannon armed B-25H offered no particular advantage over specially adapted strafers armed exclusively with machineguns. At this stage in the war, targets specifically suited for cannon attack were few and far between, and many targets that were vulnerable to cannon were also vulnerable to a battery of 0.50-inch machineguns or to bombs. Consequently, the use of heavy cannon was generally abandoned in the South-West Pacific by August of 1944."

Yes they were abandoned by mid-44...there wasn't really anything left to shoot at with the 75...

Plenty of stories by the B-25J and B-25H crewmen who appreciated the accuracy and punch of the 75. However, the recoil of the cannon was tearing the ships up.

You can hose a transport all day with .50 cal and all you're going to do is sweep the decks clear of personnel, but hit it at the waterline or the control structure with a 75mm and you've just scored a kill. Also, .50 cal doesn't do much against bridges and hardened structures.

 

[GregP]

Muskeg,

Your position is at odds with the experiences of people who were there. Paul Cherry was a crew chief with Pappy Gunn and had wonderful things to say about the B-25 with the 75 in it ... although he said it DID need a recoil absorption system in order not to scrap the aircraft after 15 - 20 shots. The originals had no recoil absorption and oval-shaped the rivet holes in short order. Paul said they could lose 15 mph of airspeed with every shot and so had to plan the attack on the way in based on airspeed and power settings.

The factory units were better from a recoil absorption standpoint and could fly and shoot a long time.

You say you are a retired artilleryman and I believe you ... but that would make you Army, right? The Air Force has no artillery .... maybe Navy? or Marines? What is you expertise in airborne weapons?

Just asking because most things in the Army or the ground-based portion of the Marines or floating portion of the Navy were NOT flight worthy ... too heavy. For aircraft you had to make it light and potent.

Believe me I fell in LOVE with the Douglas Skyraider in Viet Nam and it has EVERYTHING to do with the cannons. They could fly overhead for an hour or more and throw SOMETHING at the bad guy on EVERY pass. It was a plane that discouraged the Viet Cong from moving, much less mounting an assault ... the main reason being they never operated alone. If there was one ... there were 4, 6, 8 or more. If you attacked when Skyraiders were orbiting around, you were most probably dead, and rather FAST.

After that experience, I LOVE cannon-armed Attack planes like the A-38. Gimme'n A-10 anytime if I'm pinned down. Forget the rockets, the Skyraider could NOT shoot them close but could lay a cannon pass within a few meters accurately. Love the 20's and 30's myself, and have only shot ground-based 75's in the past. Never an aircraft-rated 75. But I'd take it without hesitation in a pinch over unguided rockets or dumb bombs.

 

[GrauGeist]

By the way, they've used a 105 (M102) aboard the AC-130 series for years. Also, I don't recall which gunship used one, but there was a 75mm used also, either the AC-47 or early AC-130. Don't recall the type.

So the idea of airborne artillery is still a very real armament option in this day and age.

 

[GregP]

Yeah, the AC-47 Puff the Magic Dragon and the AC-130 Spooky are awesome if you are watching or on the receiving end. I believe one of them even had a MIG kill when it swept in and the Vulcan sawed off a wing. A MiG-21 as I recall, but maybe not ... long time ago.

It could render a large area completely neutral and remove trees and bushes as well as a dedicated lawn care crew, and a WHOLE lot faster. I've seen 2 foot thick trees felled horizontal over a football size field in less than 30 seconds. And the AC-47 orbited for another 45 minutes in case ... nothing moved or made a sound except the AC-47 and the friendlies. The enemy certainly didn't want to get noticed if they were around and alive. I have NOT seen an AC-130 Spooky in person in combat but have seen footage.

It is nothing to mess with. The pilot alone can orbit and probably punch a hole through a WWII German submarine pen. The main item is a rapid fire cannon with effective shells that are NOT weak in power and hit at an enormous rate, mostly in the same spot if desired. If you hit a target with 200 explosive cannons shells in less than minute (from only ONE of the medium guns ... never mind the 20 mm and the 105 mm), nothing much can fight back or survive unless it is protected with 20+ inches of armor steel. Maybe the battleship Missouri ... but it WOULD sustain damage nonetheless. The sights are computer enhanced and stabilized, so hitting the intended target is a given if the aircraft is working.

The 20 mm cannons were MUCH faster than 200 rounds per minute (in the 6,000 thousand rounds per minute range). That is nothing you want to attract the attention of if you have any survival instincts at all. People who walk up and slap a lion in the face are not usually counted among the survivors.

 

[Aozora]

from Life and Times of the 341st Bomb Group

View: https://www.youtube.com/watch?v=o3xZV5q44d4

 

[muskeg13]

The 75mm cannon used in the B-25G and Hs were adapted from Army ground weapons. It doesn't matter if the cannon is fired from the ground or the air, the same laws of physics apply, particularly if they are firing the same ammunition. Just because a they are fired from an aerial platform, doesn't magically transform a relatively light 75mm cannon shell into a ship killer, not if the ship is of any size. This is barring a lucky shot that may ignite fuel or ammunition. Yes, the 75mm had some valid applications, such as taking out light skinned targets and antipersonnel work, but wouldn't bombs or multiple heavy MGs or 20mm cannon work better?

Ever heard of artillery weapons effects tables? If you're going to attack ground targets with heavy aerial cannon (not 20mm-30mm, but larger) these tables would apply. They are broken down by weapon type, ammo type and target type and give estimates of how many rounds are needed to supress, neutralize or destroy targets. I think you'd be surprised at how many rounds of ammo are necessary to effectively engage a target, and the larger and more protected a target is, the more and heavier rounds are needed. From the info in the historical links above it seems that at best aircrews might hope to get off 2-3 shots on their target runs, and these shots weren't taken while sitting behind a computer screen.

Final note, and this applies to using an aerial cannon as a bunker buster. Just prior to commencing ground operations in the First Gulf War, the Artillery Center at Ft.Sill, OK ran a series of tests to determine cannon artillery effects against the types of bunkers our forces expected to encounter in Iraq. It quickly became evident that 105mm projectiles (34lb, and exactly what the AC-130 uses) were completely ineffective, no matter how many hits were attained. The 155mm projectile (95 lb) was only marginally effective and only after a large number of hits, sometimes a hundred or more. Only our largest cannon, the 8 inch M110A1 howitzer, firing a 200 pound projectile really could be called a bunker buster, and it would take a C-5 to get this baby in the air.

 

[muskeg13]

by the way, I'm old enough to remember the days when I thought the A-7s were a Groundpounder's best friend...pinpoint bombing accuracy before the widespread use of guided munitions. That was until the first A-10 I encountered was flying so low it blew the cammo netting off of our command track. The A-10 put the Close in Close Air Support. I think retiring the A-10s now, before the F-35s are fielded is a mistake.

 

[tomo pauk]

Much as there might be arguments about the effectiveness of the 75mm the fact is that a 14lb high velocity high explosive shell is still an effective weapon against many targets; unlike the unguided rockets that had steeply curved trajectories, a flat trajectory 75mm shell could be aimed to hit a target from a distance. The opinions of a veteran who actually used the 75mm armed B-25 in combat should count for something (Jerry Scutts B-25 Mitchell at War, pages 48-51):
<snip>
The fact is the 75mm was more effective than muskeg and tomo are alluding to;

the limiting factor, and the reason the 75mm was eventually dropped from the B-25s, was the damage done to the airframe, something which the purpose-built installation of the XA-38 would have helped alleviate.

Attached is a really interesting article from an olde but goode November 1978 Airpower (16 MB):
View attachment 257368
BTW, the Tsetse Mosquitoes stayed operational until VE-Day - they were not rejected.

The attached book pages state several times that 300 lbs bombs were also dropped, about as many bombs as shells per target. The pilot's also say that they attacked mostly pin-point targets, like small bridges and vehicles, by the cannon. We know that soft-skin vehicles were equally vulnerable to multiple BMG. The target most valuable for Japanese and Allies alike were ships, and those were targeted with bombs, not 75mm. The skip- and mast-height bombers (and dive bombers) won the Pacific war, along the subs. The 75mm cannon armed B-25 entered the fray in late 1943, and was in huge minority even against other B-25s.
I stand corrected re. Tse-Tse Mossies.

What this mostly says is that the B-25s had run out of targets, which is surely an indication that the gunships had run themselves out of business because they were so effective: that "many targets that were vulnerable to cannon were also vulnerable to machineguns or to bombs" is a given - how many targets were more vulnerable to 75mm cannon when MGs wouldn't suffice and bombs were too inaccurate?

Don't think I'd like to be on the receiving end...

I'd disagree with that - the other aircraft vastly reduced the number of the ships, not the cannon-armed B-25s. In case such B-25 is trying to hit a tank, it is best for that job. But Asia-Pacific was not Russia or Low Countries, with plenty of armor-clad vehicles. Here mattered ships. With that absent, trucks and artillery were still fair game for the aircraft with multiple HMGs.

Yes they were abandoned by mid-44...there wasn't really anything left to shoot at with the 75...

Plenty of stories by the B-25J and B-25H crewmen who appreciated the accuracy and punch of the 75. However, the recoil of the cannon was tearing the ships up.

You can hose a transport all day with .50 cal and all you're going to do is sweep the decks clear of personnel, but hit it at the waterline or the control structure with a 75mm and you've just scored a kill. Also, .50 cal doesn't do much against bridges and hardened structures.

Hit the transport with few 250 lb or bigger bombs and it's game over. You will also sink a destroyer, not going to happen with 75mm.
At any rate, I'll appreciate some good data about the 75mm cannon sinking ships.

 

[Aozora]

The 75mm cannon used in the B-25G and Hs were adapted from Army ground weapons. It doesn't matter if the cannon is fired from the ground or the air, the same laws of physics apply, particularly if they are firing the same ammunition. Just because a they are fired from an aerial platform, doesn't magically transform a relatively light 75mm cannon shell into a ship killer, not if the ship is of any size. This is barring a lucky shot that may ignite fuel or ammunition. Yes, the 75mm had some valid applications, such as taking out light skinned targets and antipersonnel work, but wouldn't bombs or multiple heavy MGs or 20mm cannon work better?

A 75mm might not be able to sink ships but it can really mess up the superstructure and knock out gun positions from a relatively long range - with warships, it will destroy lightly armoured positions that .50s or 20mms wouldn't even scratch - creating enough mayhem to allow the aircraft to plant bombs in the right place; read the extract from Al Berens who used the weapon in combat. A 75mm can knock out a wider range of targets from a longer range than any number of heavy mgs or light cannon. Not saying the 75mm was a be all and end all, but it was another weapon that was well worth having because it provided another method of attack.

I think you'd be surprised at how many rounds of ammo are necessary to effectively engage a target, and the larger and more protected a target is, the more and heavier rounds are needed. From the info in the historical links above it seems that at best aircrews might hope to get off 2-3 shots on their target runs, and these shots weren't taken while sitting behind a computer screen.

A low rate of fire was a weakness in the B-25G H because the cannon were handloaded - the 75mm in the XA-38 was fully automatic (1.5 rounds per second) and aimed via a proper sight. I don't quite know why you're comparing the fixed, forward firing installation of the B-25 and XA-38 with the side firing, infra-red equipped AC-130 which uses completely different methods of attack by circling the target at medium-high altitude at night - nothing like the direct, low-level, high speed attack of the B-25s or XA-38s.

Final note, and this applies to using an aerial cannon as a bunker buster. Just prior to commencing ground operations in the First Gulf War, the Artillery Center at Ft.Sill, OK ran a series of tests to determine cannon artillery effects against the types of bunkers our forces expected to encounter in Iraq. It quickly became evident that 105mm projectiles (34lb, and exactly what the AC-130 uses) were completely ineffective, no matter how many hits were attained. The 155mm projectile (95 lb) was only marginally effective and only after a large number of hits, sometimes a hundred or more. Only our largest cannon, the 8 inch M110A1 howitzer, firing a 200 pound projectile really could be called a bunker buster, and it would take a C-5 to get this baby in the air.

How many Japanese bunkers were actually in the open and able to be targeted by heavy "bunker busting" artillery, let alone aircraft? How many Japanese bunkers were able to be knocked out by anything less than a 2,000 lb bomb? This is a red-herring because if a 155mm couldn't bust a bunker, nor could any number of .50 cal or 20mms.

Question: if the 75mm in the B-25 was truly as ineffectual as muskeg and tomo are making out then why did the USAAF and NAA bother building 408 B-25s, then follow them up with another 1,000 Hs, complete with a modified, lighter 75mm? If all that was required was a heavy battery of mgs or light cannon surely the next choice after the G would have been the solid-nose J.

 

[looney]

AAR show that the RP had about an hitpercentage of 3-4% against tanks. During the war there was a massive overclaiming from armor kills. Sources linked from this forum. Against ships hitrate muct be higher but AA denser.

 

[wuzak]

Which was best as an aerial gun - the 57mm 6 pounder with molins autoloader or the US 75mm cannon?

Greg, I don't think we can compare small calibrre cannon with large numbers of rounds and high rates of fire with the adapted field pieces.

 

[Shortround6]

"Ships" covers a rather wide range of targets. A lot of the "shipping" in the pacific, especially among the Islands, were rather small. Hitting a 8,000-14,000 ton 350-450ft freighter with a 2-3 75mm rounds is one thing, hitting a 125 ft coaster with 2-3 rounds is another.

Getting a "hit" on the waterline or on the Bridge (control space?) is also a bit chancy, as stated before it sometimes took a sub (not the most stable platform in the world) dozens if not hundreds of rounds to get the effect they wanted. Sometimes they got it in under a dozen rounds but you can walk into a casino and roll 7 at craps 5 times in row too, if you are very, very lucky.

A low level airplane is also not a very good firing platform for a single shot weapon ( or rounds fired several seconds apart.)

As far as how many were made and why?

First B-25 H prototype flies May 15, 1943,
first production plane flies July 31, 1943
First "H" is accepted in Aug 1943
The first B-25Hs arrived in the FEAF in February of 1944.
The last B-25H was accepted in July of 1944.
the use of the heavy cannon was generally abandoned in the South-West Pacific by August of 1944.

ALL of the B-25 H's were completed in just under one year and they did not see actual combat until they had been in production for 6 months or more. Production had ended before the decision to halt the use of cannon was made.

 

[muskeg13]

Aozoro: Mention of the AC-130, with it's computer assisted targeting systems, is just to acknowledge the existence of 40 and 105mm aerial cannon. The 105mm seems to be the largest practical limit for aerial cannon, but even today's 105mm has it's limitations. Manually aimed and fired WW2 vintage 75mm cannon were at a severe disadvantage...even smaller shell, slower rate of fire and accuracy limitations compared to today. FYI: Ever hear of the Piaggio P 108 and its 102mm cannon?

Bunker busting? Wouldn't bunkers have been a prime target set for the A-38 Grizzly? However, by 1944 napalm was coming into use and would have become the standard method of dealing with bunkers and pillboxes during the invasion of Japan.

Why the B-25H? Shortround 6 nailed that one. Additionally, I read somewhere that many of the cannon armed B-25Gs and Hs ended their flying days with the cannon removed and replaced by additional .50s.

 

[GregP]

Hi Muskeg,

Artillery tables? You don't use artillery tables for airborne weapons, though bombs do have a CEP like artillery shells. Even smart weapons have a CEP; it's just a lot smaller.

Artillery tables are fine for hardened targets and that is exactly what artillery is for .... hardened targets. You know that. Aircraft usually employ bombs for those targets, not cannons. Cannons are generally used offensively for airborne targets, mobile targets (trains, trucks,), and unhardened targets (normal buildings). If the aircraft is using cannons then it is most likely protecting itself or some ground asset or assets, not being offensive ... except maybe in the case of boats and smaller ships. If they are using cannons on other than small boats and/or ships, it's probably because they are out of other weapons.

I can almost guarantee that no A-10 pilot uses artillery tables. It is mostly an anti-armor aircraft, and the tactics developed for it are aimed specifically at tanks / armored vehicles and targets of opportunity. You don't use an A-10 like an artillery battery; you use it like an A-10.

Nothing wrong with artillery at all, it has a specific job and does it very well. So please don't think any of this is a knock on artillery. It isn't. But today the gunsight computes lead and impact point and all the pilot has to do is shoot when the pipper is on target and he is in range. There aren't any tables he need be aware of. Even the AC-130 has no tables for the 105. It has a lead and range computing gunsight. When the target shows up in the sights, shoot. They don't hand around and do BDA either, they shoot and scoot.

If the 105 or whatever cannon is ineffective against a target, they hose down the area until our assets can escape, and then fly away if able. Even if it can't penetrate a target, it certainly CAN make the vicinity uninhabitable by enemy troops while we retreat and think about plan B. Nobody is trying to replace an artillery attack with an airborne cannon-equipped aircraft.

When they think of replacing artillery with aircraft, they think about bombs, not cannons. The Yamato could fire a 3,000 pound (1,360 kg) shell a distance of 26 miles (42 km). Only a bomb of suitable size delivered by an aircraft could hope to do similar damage. Well, maybe a warhead on a missile, but I was thinking more of WWII. The Germans DID use the V-2, but it was NOT anywhere NEAR accurate enough to replace a well aimed bomb.

 

[tomo pauk]

<snip>
When they think of replacing artillery with aircraft, they think about bombs, not cannons.
<snip>

Summed it right there - the purpose of the B-25 and the like was to drop bombs beyond artillery's range. Not to lug a, by standards of ww1 and further, smalish cannon.
Thanks, Greg.

This is what it took some time to kill a strongpoint - M12 SP arty, featuring the 155mm gun: