The principal "VVS" aircraft during World War II were the Ilyushin Il-2 "Shturmovik" armored ground attack monoplane and the series of AS Yakovlev OKB-115 designed single-engined fighters, beginning with the Yak-1 and its successors. The Il-2 became (at 36,183 built) the most produced military aircraft of all time, with the four main versions of Yak fighters (the Yak-1, −3, −7 and −9) being slightly more numerous, at a total of 36,716 among them. These two main types together accounted for about half the strength of the VVS for most of the Great Patriotic War. The Yak-1 was a modern 1940 design and had room for development, unlike the mature 1935-origin Messerschmitt Bf 109. The Yak-9 brought the "VVS" to parity with the "Luftwaffe" and eventually allowed it to gain the upper hand, until in 1944, many "Luftwaffe" pilots deliberately avoided combat with the last and best variant, the out-of-sequence numbered Yak-3. The other main "VVS" types were Lavochkin fighters (mainly the La-5), the Petlyakov Pe-2 twin engined attack-bombers, and a basic but functional and versatile medium bomber, the Ilyushin Il-4.