Did it apply to British crews as well?

Yes, unfortunately I can't comment on the USAAF tours, but in the RAF a first tour was 30 missions. This would be followed by a spot of leave and a six month stint flying non-operational duties (flying as an instructor and so forth), followed by a subsequent tour of 25 missions.

The definition of a mission wasn't always clear cut. If you were forced to return to base without entering hostile airspace, this wouldn't count at all.

Furthermore, missions like dropping mines in the North Sea, or missions that didn't involve entering German airspace would only count for 1/3rd of a mission. These were called 'milk runs' and generally brand new crews or crews almost at the end of their tour tended to get more milk run missions, for obvious reasons.

Apart from that, milk runs were pretty unpopular as you needed to complete so many of them to constitute a tour.

The Pathfinder squadron required 45 missions for a first tour in addition to any previous missions - and most of those selected for pathfinder duties would often be a fair way through the first tour to begin with (Bruce Lewis gives an example of a crew who flew all but 2 missions before volunteering for 617 Squadron, and that meant starting from zero all over again). I have to say that I am somewhat in awe of the men who volunteered for Pathfinder and 617 duty, considering the chance of surviving an ordinary tour of 30 missions unharmed was only 24%.

Aircrew who refused to fly had their records marked with LMF for Low Moral Fibre. They were treated very poorly - stripped of rank and given the most menial and distasteful tasks, cleaning toilets etc.

Also of note is the youth of these men, which is all the more astonishing given their responsibility and the fact that they were charged with operating the most expensive and technologically advanced machines yet devised by mankind. One aircrew earned the nickname 'Grandad' aged 24!

I'd recommend reading The Nuremberg Raid by Martin Middlebrook and Aircrew by Bruce Lewis.

Yes, in 1943 the USAAF in Europe determined that heavy bomber crews would be rotated home after 25 missions over enemy territory after first trying a one-year tour or a tour consisting of a pre-determined number of flight hours. Tour lengths were determined by theatre commanders, so the policy in Europe was different than Asia, the Mediterranean was different than the Middle East and so forth. It also varied by type of aircraft-fighters, light bombers, medium bombers, heavy bombers. For the most part, I will cover the European Air War, since that's the area your question covers.

In late 1943 and early 1944, the P-51 Mustang went into operation in large numbers as a bomber escort. This was critical, since the P-51 could accompany the bombers for the duration of their mission (previously used escorts such as the P-47 Thunderbolt and the Supermarine Spitfire didn't have the necessary range and had to turn back long before the bombers reached their targets in Germany). As a result, bomber losses began to decline, and the USAAF increased the tour to 30 missions, then by the summer of 1944 it was increased again to 35.

Bomber crews who completed their tours were generally re-assigned to training bases in the US to impart their knowledge on the next generation of crews or assigned to non-combat roles in the theatre where they were serving. There were some exceptions. Some crewmen would re-train on new equipment. Robert Morgan, the commander of the Memphis Belle, trained on the B-29 and commanded a squadron in the Pacific (he flew 26 missions over Japan). There are also examples of bomber pilots who completed their tours re-training in fighters and returning to combat as fighter pilots.

If you're looking for a great history of the air war, particularly of the 8th Air Force which was the USAAF heavy bomber force stationed in England, pick up Donald Miller's Masters of the Air. HBO is using it as the basis of their next WWII mini-series in the vein of Band of Brothers and The Pacific.

On this specific question, here is a 1968 document from the Air Force about crew rotations in World War II and the Korean War.

Robert McNamara covered this in "The Fog of War," and essentially confirmed the fact. At the time, he was a statistician for the AAF. He found the abort rate was near 20%.

After analysis, they realized the loss rate was 4% per sortie, tour was 25 sorties, and pilots found reasons to not go over the target.

Col. LeMay addressed his pilots and basically threatened court martial for any flight crew that didn't follow him (in the lead plane) to the target. The abort rate dropped.

That said, a 4% rate over 25 missions doesn't necessarily come out to a 24% loss rate overall for a tour, but it's significant enough to have an effect on morale.

I highly recommend "Fog Of War" to all the readers here, rarely do we get to hear the minutiae of multiple wars from such an intrinsic source.

The consensus in this discussion is that WWII bomber crews were rotated home after 25 combat missions. Perhaps this only applies to the European theater of the war. I'm sitting here with raw WWII data - my father's complete flight record from April 7, 1941 (training as a cadet before Pearl Harbor) through 1950 as a member of the Reserve USAF.

He flew B-24 bombers in the China Burma India (CBI) side of the conflict. From February to September 1943, he was a co-pilot out of Gaya and Bishnupur India as a member of the 10th Air Force, 7th Bombardment Group, 436th Squadron. Twenty-five combat missions are recorded with targets such as the Mahlwagon rail yards in Rangoon, the Gotkeik viaduct, Mytinge bridge, Pyinmana bridge, Thilwa oil refinery, and various other Japanese installations in Burma.

In September 1943 he was transferred to the 14th Air Force, 308th Bombardment Group, 373rd Squadron under General Chennault at Kunming and Yangkai airfield. Here, he piloted numerous supply runs over "The Hump" and piloted an additional twenty-four combat bombing and mining raids on Japanese shipping in the China Sea, Hong Kong, along the Burma Road, Haiphong, Chiangmai, Hanoi, Lampang, Shanghai, and actually reached targets in the Philippines. He finally returned home on February 1944. So, a few hundred flights and 49 recorded combat flights, although dad always said it was 62.

I'll share a few of his photos in India and China: http://imgur.com/a/Y2d4s