In WWII, fighter pilot kill claims were generally exaggerated by about a factor or 5 to 10(Edit: to a degree ranging from very substantial to very extreme), when compared against enemy loss numbers in records available after the war. This is true for all countries participating in the war. Even with gun camera footage, a group of pilots tended to double, triple and quadruple count planes that did go down. Plus many planes that were shot at and hit did not suffer serious damage and recovered after diving out of the combat area.

Most countries had fairly strict rules designed to prevent overclaiming. However these rules were frequently flouted and basically ineffective at combatting the problem. WWII Pilots under the pressure of combat were either unwilling or unable to prioritize getting the counts right for intelligence officers in charge of tracking enemy losses. The Air staff officers were aware of overclaiming as a problem, and discounted claims to some degree, however they had little information to go on. Sometimes the exaggerated claims were believed by the staff officers and they made incorrect decisions as a result. Undiscounted claims were sometimes used as propaganda, but the motivation for overclaiming was not primarily propaganda, it was just a property of combat conditions in air units.

Edit: This post got enough attention that I'm feeling guilty about the sourcing of the assertion in the first sentence. I'm going to make this much more vague until I can put a more substantial source behind the general size of the overclaiming problem.

fighter pilot kill claims were generally exaggerated by about a factor or 5 to 10

Really? Overclaiming does occur and happened on all sides, but that seems extremely high. Do you have any sources for that?

I can discuss the kill claims of the German Luftwaffe as they're my field of 'expertise' and are usually under the most scrutiny in popular history. German aces were by far the most successful, with some of the best reaching many times more kill claims than their most successful allied counterparts. Post-war, these claims (and the methodology for confirming them) have been scrutinized against the records of Allied pilots. The Luftwaffe was found to have arguably the strictest policy for verifying kills which required microfilm footage or a witness - or two, in the case of enlisted men, to confirm "that enemy aircraft was downed, not damaged, or the pilot was seen to have bailed out". (Spick, 1996). When Russian Air Force records were declassified after the fall of the Soviet Bloc, Luftwaffe kills (particularly those of the most successful pilots, who naturally came under the most suspicion) generally stood up better to cross-referencing than those of allied pilots (Claasen 2004, Spick 1996).

So why, in the case of German pilots, were their kills so high? The short answer is that targets were available. As the war progressed the Luftwaffe was increasingly outnumbered and outclassed, and the Allies achieved air superiority and effective air supremacy from 1944 onward. When German pilots flew sorties, they would meet the enemy. Allied pilots could fly a whole tour and rarely even see an enemy fighter, if ever. (Shores, 2005).

One aspect of over claiming that persisted in the Luftwaffe was pilots both claiming the same downed plane. This was common in that late stages of the war when fighters spent much of their time attacking Allied bomber formations, and two (or more) pilots attacking the same bomber would often claim it as shot down. This usually wasn't addressed or rectified by the chain of command. I recommend Manrho & Putz's Bodenplatte (2004) for more info, where they speculate that this was for morale purposes in the case of bombers claimed by German pilots - so I think your suspicion of over claiming for the purpose of propaganda is correct. The book also describes the situation in Europe nearing the end of the war, and how the performance of German pilots, (often lauded IMO as the "best of the best" when popular history and Top Gun narratives collide in the minds of enthusiasts) actually performed worse and worse as the war claimed the best pilots and teachers.

Speaking less towards pilots and more towards bomber aerial gunners, kill counts were exceptionally inflated by the bomber boys as well as fighter pilots. Donald Miller’s Masters of the Air described several fairly logical explanations for this on the part of the crews and ground staff. Perhaps greatest of these was the tightly grouped protective formations of B-17s and B-24 bombers that enemy fighters encountered. Multiple gunners on multiple planes would all fire on single enemy planes as was intended by the defensive strategy. If indeed, the attacking fighter was hit and started a terminal dive, multiple gunners would all claim the kill, sure that their bullets dealt the killing blow.

On top of their individual claims, the very nature of (in this case German) attacks were also deceptive. Fighters would commonly make a firing run before inverting (flipping upside-down) and diving out of the bomber’s way to avoid collisions. As they did so, German fighters tended to leave a heavy black exhaust trail. This exhaust was commonly taken as critical damage and claimed by those same (multiple) crews as kills or probables.

Added to these issues were those of every conflict. Adrenaline, stress, panic, and the release and euphoria of survival after a mission could all drive numbers up. Thirty fighters could easily become a hundred plus in the minds of a crew that was dealing with battle shock (what we know as PTSD today).

As to why these numbers were published near and far by both the press and military, look no further than propaganda. The strategic bombing campaign started in 1940 by the RAF and 1942 by the USAAF was horribly ineffective early on and the returns were not justifying the casualties. Without some ‘good’ news to go out, Generals worried that the campaign would be called off before it had any chance to achieve something real. It’s widely accepted that most of the intelligence officers and general air force staff knew the kill counters were vastly inflated, but they needed good news more than they needed accurate information being fed to the general public. So they let the numbers stand not only to encourage the public, but to justify the operation to political leaders and war planners as well.

In summary, when it came to strategic bombing missions, the issue of over counting was less about personal gain, and more due to a series of understandable mistakes along with propaganda based purposes.

Sources

Miller, Donald L. Masters of the air: America's bomber boys who fought the air war against Nazi Germany. Simon and Schuster, 2006. Kennett, Lee B. A History of Strategic Bombing. C. Scribner's Sons, 1983.

Not strictly about pilot kill counts, but I remember reading a fragment of an interview to Otto Carius (link, in Spanish), the German tank Ace. He was asked about his kill count, this was his answer:

Interviewer: According to historians and the press and Nazi Propaganda, you destroyed about 150-160 enemy tanks. Is this figure real?

Carius: This figures are exaggerated. I destroyed 100-110 tanks at best, and the number was inflated by the German propaganda ministry. Kurt Knispel's numbers? Pure propaganda. I had the chance to meet some of his crew in Russia and they admitted themselves that they didn't destroy more than 60 or 70 tanks.

I don't know how reliable this is, as I've never seen Otto Carius mention it anywhere else. However, the guy does have pictures with Carius in the blog, so the interview at least happened.

Many replies below have asked for sources and confirmation, and I will try to answer these.

In recent years there has been a lot more effort put into cross-matching kills claims against recorded losses, ie, comparing the reports on both sides. And indeed overclaiming in the region of 3-1 to 5-1 is common, with 5-1 being virtually a minimum standard figure.

Ideally, the more judicious air forces required third party confirmation of a 'kill' (the Germans and Soviets), either a crashed aeroplane of confirmation by a combattant not known to the pilot (eg, not a squadronmate). For the Germans, if two men shot down a plane, it would usually be awarded to just one.

The Allies tended to rely more on after-combat reports, which were sifted by intelligence officers and kills awarded as outright (generally, the plane was seen to crash and verified), 'probable' (aircraft damaged and apparently unflyable, but not seen to crash), 'damaged', with 'shares' being awarded as well. The Allies more often fought over enemy territory or did not have the ability to verify a kill by taking possession of a crashed aeroplane. Combat footage would also be reviewed, but gun cameras and film were not always available.

The real basis for all this overclaiming is the extreme confusion and speed of air combat. Air combat between fighters was usually something like a pub brawl with someone turning the lights on and off. Only a tiny minority of pilots were really cool and in control in combat, for most it was desperate survival, maneuvering hard, often almost blacking out under G forces, and shooting at anything that might be hostile. Planes could be remarkably tough and survive being shot at my multiple enemies, an evasive maneuver could look a lot like a loss of control, and even engines could 'sputter' and spew out smoke, especially if they had carburettors affected by G forces. In combat involving bombers, the situation bordered on the ludicrous: a Messerschmitt making a high-speed pass through a 'box' of B-24s might be shot at by twenty or more gunners, and if it was damaged they'd all claim it in good faith.

You ask what stopped scores being inflated or 'made up', and ultimately there was not a lot to stop this if a pilot really wanted to. In reality, few pilots would want to earn a reputation for lying, or even have their claims doubted. There are at least two cases I know of of pilots pretty much just 'making shit up'.

Sgt Satoshi Anabuki (50th Sentai) claimed two P-38s and three B-24s in October 1943, after losing his squadron and later crash-landing on a beach. Not only were there no witnesses, and this does not match any Allied loss reports, there were no P-38s (a very distinctive aircraft) anywhere near the area at the time, and B-24s had been bombing Japanese positions almost with immunity (the JAAF's Ki-43 was armed with just two machine guns). Could it be that a veteran pilot (Anabuki had a solid record and ace status from well-documented combats) who lost his men in a vital air battle, and later lost his way and crash-landed, made up a good story? Especially since the Japanese at the time in Burma were very hard pressed? And the Japanese forces in particular were susceptible to a culture where proving one's bravery in the quasi mythical samurai tradition was more important than ensuring your commander shad accurate intelligence.

The other case involved the US Navy, and operations over Rabaul. Robert Hanson of VMF-215 was flying Corsairs as the Allies made a series of concerted attacks on what had been a long-standing thorn in the Allies side, the Pacific base of Rabaul. Competitiveness among Navy and Marine pilots was rife, and Hanson's claims were doubted. A skeptical squadronmate determined to stay with Hanson (he routinely 'lost' his wingmen, then claiming multiple victories, unwitnessed, in ensuing combat) and was amazed at the effort Hanson put into losing him, and his icy resentment at being scritinised. Many of Hanson's claims were like Anabuki's big day: unwitnessed claims of multiple victories, often involving damage to or crashing his own plane. There is more to this story, which is detailed in Bruce Gamble's excellent Target: Rabaul, but Hanson was soon killed and remains a Medal of Honor recipient and a hero.

If this is not the right place to ask this question - Please delete this comment.

My question related to this topic is that why did Finnish pilots during the WW2 had such high kill rate?