B-17G with a 20 mm cannon onboard!

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Change of topic for a bit, but I was surprised that the B-29 had a rear mounted 20mm plus two .50 cal machine guns. Then after WWII the 20mm was removed leaving only the twin .50 cal in the rear. Now I see that the AF was trying the 20mm out in the B-17 as well
 
I know that a few Luftwaffe bombers carried 20mm guns in defensive positions also, specifically the FW 200 had either an MG/FF or later a MG 151/20 mounted at the front of the gondola. I think with the FW 200 the intention was more for suppressing AA fire from ships but it was surely used as a defense from fighters too. Also the Japanese Betty carried a 20mm in the tail position. I wonder how they overcame the traversing and recoil problems that kept the 20mm from being effective in the B-17? I'd never heard of the 20mm installations in B-17's before, that's really interesting! Learn something new every day I suppose...
 

The key difference is that the German and Japanese bombers were designed and structured for the recoil. The B-17 was designed fo a .30 caliber in a ball socket and nothing significant was done until the YB-40, then the B-17G, to take recoil in the nose compartment.

All the mods to 'cheek' and nose mounts in the B-17E were done in the field. The B-17F was first to have a factory mount in front for .50 caliber in either cheek, but the mount for the bombadier was also done in the field - and that was the basis for the 20mm 'attempt' - which beat hell out of the nose and deemed dangerous.
 
I've also seen pictures of a field-modded B-17 that had a set of twin .50's cut into the plexiglass nose. It seems like the recoil and vibration would be close to that of a single 20mm so maybe it wasn't deemed effective either.
 
Multimetal said:
I've also seen pictures of a field-modded B-17 that had a set of twin .50's cut into the plexiglass nose. It seems like the recoil and vibration would be close to that of a single 20mm so maybe it wasn't deemed effective either.
Click to expand...

Most B-17F's in the 8th AF had that twin mount as a field mod before the chin mount came with all factory B-17G's..
 
After several years back to this topic. 20 mm cannon in tail story at 2nd BG is getting more and more interesting. So far we know that on Aug. 29, 1944 at least 3 Forts were equipped by this experimental gun. S/N 42-38096 (Big Time, 20 mm shells found at the crash site), 42-31473 (My Baby, 20 mm shells found at the crash site) and 42-31885 (20 mm shell found at the crash site plus pics from the crash site showing this gun).
But today I spent some time on the Narrative Mission Report and found something interesting again. In the point 6 of the Daily Operations Report there's written:
0.50 API&T - rounds carried 235.400, rounds expended 21.730, rounds lost on missing & destroyed aircraft 57.240.
And now the Holy Grail - 20 mm - rounds carried 1200, rounds expended 100, rounds lost on missing & destroyed aircraft 600...
So, 2nd BG sent out 28 machines, some of them carried 20 mm cannon in tail and half of them was a among those lost 9 machines (1200 20 mm rounds carried and 600 20 mm rounds lost with destroyed machines). So it means that 2nd BG itself had a significant number of the machines with this "toy".
How many? A good question - Hispano M2 20 mm cannon had a 60 rounds magazine. Should each of those 9 Forts that were shot down have one magazine only, it would make 540 rounds lost with the machines. Report says 600 rounds lost so it's clear that those machines had to have at least 2 magazines. And I also think, that because of weight, that was it. 2 magazines make 120 rounds per machine. B-17s were tail-heavy (F version for sure more than G) and I doubt that they would overload the rear part more than necessary needed. 600 rounds lost with the machines would then mean 5 machines lost with this gun and also other 5 machines with this gun that made it back. So on my opinion 10 (!) B-17Gs from the 2nd BG were equipped by 20 mm cannon in tail on that day. I may be wrong but my interrogation goes on.
 

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I have to correct my calculations above. I friend of mine told me that 20 mm cannon fitted in B-17 tail didn´t use the drum magazine but the belt feeding system. So there's still is a question have many machines from the 2nd BG were equipped by cannon on August 29, 1944 but taking into consideration that at least 3 of the machines that crashed here (42-31473, 42-31885 and 42-38096) had cannon and the fact that this group took of with 1200 20 mm rounds and that 600 rounds were lost with the machines it would mean that at least 3 other machines with 20 mm cannon got back to the base on that day.

Anyhow, here is the list of the machines with 20 mm cannon in tail that I have so far:

41-9023 (E version, 91 BG) - B-17 IN ACTION BY STEVE BIRDSALL SQUADRON/SIGNAL PUBLICATIONS with a note that the project was abandoded because of popping rivets during recoil
42-30267 (F version, 2 BG + 97 BG) - source 5TH BOMB WING HISTORY OF AIRCRAFT ASSIGNED - DICK DRAIN
42-31473 (2nd BG) - 20 mm shells found at the crash site
42-31590 (2nd BG) - source Mr. Steve Birdsall
42-31885 (99th BG, later 2nd BG) - picture of the cannon from the crash site
42-32046 (483rd BG, later 99th BG)- source The B-17 Flying Fortress Story by Roger Freeman, note on page 174
42-38087 (2nd BG)- mentioned in the 2nd BG database
42-38090 (97th BG)- The B-17 Flying Fortress Story by Roger Freeman, pic of the tail on page 44, note on page 45
42-38096 (2nd BG) - 20 mm shells found at the crash site
42-97438 (2nd BG)- source Mr. Steve Birdsall
42-97490 (2nd BG)- source Konečná zastavka Slovensko (Final Destination Slovakia) by Peter Kassak, pic of the cannon,page 84
 

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wow...I missed the details in pic. #3 (I'm on a smart phone), but when I expanded the image, I also noticed that it's an earlier B-17 (pre-G model) and it appears that the gunner's position has custom armor around the gunner's canopy.
 
It reminded me one episode from a good book

The voice from the tower was our salvation: "Return to your parking spots, all daylight bombing has been canceled, by order of General Hoyt S. Vandenberg, Chief of Staff USAF."
Not long afterward the man himself visited B-29 units in search of better ways to deal with the MiG menace. I have a color slide of the four-star general surrounded by B-29 officers while listening to an enlisted tail gunner...
One of the first things General Vandenberg said was, "I want to talk to a tail gunner." One gunner eased forward. Vandenberg asked him, point blank, "Would you guys like the 20mm reinstalled in the tail?" They all replied in the affirmative, but I don't think it was ever done, probably because the trajectory of the shells fired from the 20mm cannon was completely different from that of the bullets from the 0.50-inch machine guns, which made aiming in combat even more difficult.

(J. McGill, Earl.. Black Tuesday Over Namsi: B-29s vs MIGs—The Forgotten Air Battle of the Korean War, 23 October 1951 . Helion & Company Ltd.. Kindle Edition. )
 
Did you reach any consensus on whether or not a 20-mm gun was worth it? I would assume the inlined Me-109 to be not very tough, and to my knowledge, at the altitudes the B-17s flew, the FW-190s performance went down hard, no?

And what were the differences in hit probability, given the lower RoF?
 

As written in one of the books this gun had more psychological effect for the bombers crews than the destructive effect. Lower rate of fire, limited field of fire, hits accurancy and a different trajectory. Should it be an effective improvement more machines would be modified. So far I have found 1 Fort with this gun at 8 USAAF and this project was abandoned and 10 Forts at 15 USAAF. My estimation of the number of the modified machines at 15th is about 20 pcs.
 
Dimlee said:
but I don't think it was ever done, probably because the trajectory of the shells fired from the 20mm cannon was completely different from that of the bullets from the 0.50-inch machine guns, which made aiming in combat even more difficult
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out to 500 yds the trajectories were never more than a few inches apart. Beyond that range trajectory and time of flight did start to differ more. The Hispano and the .50 Browning were probably one of the better match ups as far as trajectory and time of flight go.
 
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