According to this wiki in 1940 it was 97.8% white.
https://en.wikipedia.org/wiki/San_Bernardino,_California
In 1940, California was 95.5% white, with Hispanics being counted as white. 1.8% were black and 2.7% were other race.
http://californiawatch.org/dailyreport/census-1940-calif-led-country-education-15595