I don't know a great deal about the history of US plastic kits, but here in the UK, the first plastic kits were actually pre-war. They were made in the mid/late 1930s by the company FROG (better known for their 1960s-70s series of plastic aircraft kits).
As, at the time, FROG mainly produced flying aircraft models, they called their model kits "Penguins" as they couldn't fly (seriously!).
They were 1/72 in scale and made from injection-moulded plastic. A variety of subjects were produced - mostly British aircraft of the period, such as the Gladiator, Fury, Wellesley etc.
The "Penguin" kits were produced in a cellulose-type plastic which warps and deteriorates badly with age. Most Penguin kits still in existence are so badly warped as to be unbuildable, but they are so valuable and collectable now that nobody
in their right mind would want to build one!
I don't know if "Penguins" were the first ever plastic kits, but they were certainly the first ones produced commercially on a large scale!
Here is a website with plenty of information on the Penguin series: http://www.frogpenguin.com