Since you did not state that the ARF has to be R/C (....), the real first ARFs would have to be some of the control line models by Jim Walker. like the Fireball and the Firebaby:

[link]http://www.americanjuniorclassics.com/[/link]

They pioneered the concept of ARF models. The Fireball could be built in as little as 6 hours and had a fully pre-carved balsa fuse and all parts were ready to assemble.

The Firebaby could be built in a couple of hours.

There were any number of lightweight ARF and RTF control line models in the 1950s, some made from very thin aluminum.

I remember lusting after the magazine ads for the Testors .049 powered low wing RTF R/C model (looked like a Cherokee, but was called a Shyhawk for some reason) in the late 1960s.

I seem to remember that Mattel gets the nod for the first parkflyer electric ARF, for the R/C version of the programmable FF electrci Super Star from the 1970s.

I do think Lanier gets the credit for mainstreaming plastic and foam ARFs in the early 1970s.

The famous Hobie Hawk (made in the USA!) sailplane could be obtained as an ARC, and ARF and a RTF, with a Kraft 2 ch brink pre-installed. The Hobie Hawl was quite advanced for its time...curved and molded presheeted with 1/64 ply foam wings, glass and plastic fuse.