UBC was indeed initiated in 1927, but was only really used in the western United States. It was based in California, if memory serves.

The first national level building code was formed by BOCA (Building Office and Code Administration). The compiled code was first released in 1950, a full 35 years after the inception of the organization.

There are numerous state and regional codes that predate all of the aforementioned, of course.

But the first cohesive compilation of truly national code requirements was indeed in 1950.