Query: When did it become NBC's first affiliate?
Context: In the 1930s, the experimental station W3XE, owned by Philco, became the first television station in Philadelphia; it became NBC's first affiliate in 1939, and later became KYW-TV (CBS). WCAU-TV, WPVI-TV, WHYY-TV, WPHL-TV, and WTXF-TV had all been founded by the 1970s.