Gen. MacArthur's big red Parker pen was used to sign the Jap surrender documents in Tokyo Bay, if memory serves. Parker had an ad about it a few years ago.
Besides Parker (with various models), Sheaffer was probably popular, and Waterman. My father had a Sheaffer fountain pen when I was a kid, and may have carried it in the war. Mont Blanc was also making pens, but mostly not for US GI's.
The first I recall about ball points becoming popular was in the 1950's, when Art Linklater (sp?) was advertising Paper Mate pens on his TV show. I think it was called, "People Are Funny", one of the first shows that I recall.
As late as 1960 or so, kids here were trained in school to use fountain pens.
I own some modern fountain pens and a couple of Parker 75's that date from the 1960's. But I seldom use them. They're just too messy to clean, and when the nibs wear down, are very hard to replace, as the pens have been out of production for years. The Parkers that I used in elementary school (7th grade) were either Model 51 or 61.
The WW II GI used pencils or whatever fountain pen he could afford. That'd also be so for our allies and their European enemies. The Japanese may have used some special brushes or calligraphy pens.
This is a good question. I hope that someone can shed more light on the matter, without posting copyrighted pics. If I understood the new standards on the board, a link to a suitable photo is still okay.
Look for ads about pens in old issues of, "Life", "National Geographic", etc. Libraries often have them. Parker may have the MacArthur pen on their site. They've featured it in the past. I think it was called a Big Red. Something like that...
T-Star