Project studies by both firms started in 1964, at about the same time the US Army started a project that resulted in the TOW missile. Unlike TOW, which entered service in 1973, the development and testing phase for HOT took considerably longer. The design goal was to produce an antitank missile that could be fired from both vehicles and helicopters; that employed the SACLOS guidance system instead of the less reliable MCLOS system used by the SS.11; had a longer range combined with a better minimum engagement range; had a higher missile speed than the SS.11 resulting in a shorter flight time; and packed in a sealed container that also served as the launcher.