The Lotus 31 was a racing car produced by Lotus for the new 1-litre Formula 3 introduced in 1964, replacing the expensive Formula Junior. The chassis was a multi-tubular spaceframe, of similar design to the 1962 Lotus 22 Formula Junior. A 997 cc Ford Cosworth 109E with Single Choke Weber or SU carburettors produced 97 bhp (72 kW) at 8000 rpm. The 31 had little success against the more advanced monocoque cars that dominated in 1964. Records show only 12 were built in 1964/65.
In 1966 19 cars were built with the Chassis 22/F3. The relative simplicity and low price of the car made it an ideal training car. Motor Racing Stables at Brands Hatch ran 4 cars to graduate students from Lotus Cortinas. Jim Russell ran the cars at Snetterton, later being recognised as a club under the RAC. JRRDS ran 5 31's at Brands Hatch BARC Trophy Races in July 1967.