A can remember buying Airfix kits in Columbus, Ohio at least as early as 1962 - maybe earlier.
On High Street in downtown Columbus there was a store called Hall's Hardware, and it had a hobby shop in the basement. To a 12-year-old model enthusiast (who had to ride the city bus to get there) that place was heaven on earth. You walked down a creeking flight of stairs from the hardware store, and at the foot of the stairs you were confronted by all sorts of wondrous things: model airplanes hanging from the ceiling, cases of brass HO locomotives, and shelf after shelf of plastic kits of all descriptions. And a rack of Airfix 1/72 airplanes in plastic bags. I particularly remember the bright red Fokker Triplane and the bright yellow Tiger Moth.
Hall's also had at least some of the bigger Airfix kits - the Wellington and Halifax, for instance. (I vividly remember the intriguing interior detail of the Halifax - even if the standing flight engineer figure did look like a zombie.) I suspect the new Airfix kits were getting to that store within a few months of their release in England. I think Hall's was also selling some, at least, of the Airfix railway models.
My earliest recollection of Airfix ship kits dates from about 1963. My memory is a little hazy, but I think I remember my mother buying me the
Hood and
Bismarck kits when we were visiting relatives in Lima, Ohio at about that time. If a store in Lima had them, they must have been pretty widely distributed.
At about the same time - 1964 or thereabouts - the Airfix kits started turning up in American boxes. For a while the labels said "Airfix Corporation of America." Then there was a period when they were labeled "Airfix by Craftmaster." (Craftmaster was a manufacturer of paint-by-number sets, among other craft items.) Still later, they appeared under the MPC lable. My recollection of the dates is hazy, but I think I remember peddling my bicycle to a store in the adjacent suburb and bringing back an Airfix Corporationa of America B-17 when I was in junior high school. If my memory is correct, that must have happened between 1963 and 1965.
Another British firm I remember from those days - and in fact from a little earlier - is Rosebud Kitmaster, which made a range of OO scale locomotives. The ones I remember in particular were Stevenson's Rocket (in bright yellow plastic) and, oddly enough, the American Civil War engine The General. Rosebud also made quite a few more modern British and Continental European steam and diesel locomotives, which looked strange on my miniscule HO layout. As I remember, they were sold not only in hobby shops like Hall's Hardware but also at F&R Lazarus, Columbus's big downtown department store. It had a pretty comprehensive electric train department - especially at Christmas time. I'm pretty sure my mother bought me some Rosebud kits when I was in grade school - i.e., before 1963, and maybe as early as 1957 or 1958. Those Rosebud kits, as I understand it, now bring astronomical prices on the collectors' market.
I don't recall bumping into Frog kits until a good bit later. My earliest recollection of them at the moment is when they started to appear in the U.S. under the "Air Lines" label. (Air Lines had a fancy logo with the word "FROG" in the middle of it.) I think that must have been in the latter part of the sixties - but maybe a little earlier. I remember picking up some odd-looking Air Lines kits, like the Blackburn Skua, at the local drugstore - so they must have been pretty widely distributed. I don't recall seeing any of them in British boxes or bags until the mid-seventies, when I was working in a hobby shop. But I suspect they were around before then.
These are the random - and probably defective - memories of a 54-year-old brain. Surely the Airfix Corporation has records of when, and by whom, its products were distributed in the U.S. At least two books about the history of Airfix have been published; the second one, I think, is still fairly readily available. Maybe they have some of the information you're looking for.