As the need arose to replace the Yakovlev Yak-28 Brewer making up the backbone of the Soviet Air Force's tactical bomber fleet since 1960, the Sukhoi Design Bureau began development of a twinjet tactical bomber known in-house as the T-6. The first prototype (called T6-1) flew in June 1967, featuring delta wings and four lift-jets buried in the fuselage to improve field performance. However, the weight penalty imposed by the lift-jets was deemed unacceptable and the aircraft was radically reworked to feature variable-geometry wings, becoming the Soviet counterpart to the General Dynamics FB-111. The resulting T6-21 entered flight testing in May 1970, subsequently entering production and servi