 Although certified as Series 100s, 200s, 300s, etc., the different 707 variants are more commonly known as Series 120s, 220s, 320s, and so on, where the "20" part of the designation is Boeing's "customer number" for its development aircraft.Announced in July 1957 as a derivative for shorter flights from shorter runways, the 707-020 first flew on November 23, 1959.Its type certificate was issued on June 30, 1960, and it entered service with United Airlines on July 5, 1960.