13. This tank was the culmination of a successful series of tanks, all of which pioneered and developed the use of the Christie suspension. This tank in particular served well heading into World War 2, but was phased out later.
From Quiz Tanks of the Bear
Answer:
BT-7
The BT series was the first to use the Christie suspension system, developed by American John Christie, whose designs were disregarded in his home country. However, successive designs more than proved the Christie suspension. By the time of the BT-7, the series had created a mobile, fast, and moderately powerful tank. It was intended as a light tank, and so was understandably lower in armor protection. It more than outperformed the Japanese tanks, and easily defeated German models in the Spanish Civil War. However, in the war itself, due to the amazing pace of armor development, the BT-7 could no longer perform, due to its poor protection.