Staff Writer (Updated: 4/13/2016):
The primary conventional threat to Soviet air defense of the 1950s became the Boeing B-52 "Stratofortress" - a mammoth jet-powered heavy bomber introduced in early 1955 with the USAF. Flying high and carrying a great warload of bombs, the aircraft proved herself a capable platform and manufacture ultimately netted 744 of the type (the design remains active even today - 2016). At this point in Soviet aviation history, the primary interceptors on hand were the Sukhoi Su-9 and Su-11 which, it was realized by Soviet authorities, did not hold the intercepting capabilities required of them to counter the B-52 threat and others emerging from the United States and Britain.
From the Sukhoi "T-49" prototype arose the" T-58" which relied on an ultra-streamlined form that included swept-back wings and a side-by-side twin-engine configuration. The pilot sat aft of a radar-equipped nosecone with generally good views of the area surrounding his airplane. The twin engines were aspirated by side-mounted intakes and exhausted at the tail through circular ports. The wing mainplanes were set about midships and low-mounted along the fuselage sides.
The prototype achieved first-flight on May 30th, 1962 and this form, following some alterations to make her service-capable, was selected for serial production in February of 1962 carrying the designation of "Su-15". From August 1963 on it was involved in the usual service tests required of Soviet aircraft but the expected production lines Novosibirsk (already committed to the Yakovlev Yak-25) were contested and served to delay entry of the Su-15. As such, its introduction did not take place until 1965 and, once recognized by NATO, it received the codename of "Flagon-A".