The preliminary version of the 659 cc engine was first seen in the "i" Concept test car introduced in 2003, and used Mitsubishi's Smart Idling system which turns off the engine automatically when the vehicle is stationary, and can restart it within 0.2 seconds. So equipped, Mitsubishi claimed the prototype was capable of fulfilling the "three litre initiative" for gasolineengines, meaning fuel consumption of no more than 3.0 litres per 100 kilometres (94.2 mpg, 78.4 U.S. mpg).
The larger 999 cc capacity of the development engine was outside the limits of the kei class in Japan, but was introduced in the second generation of the smart fortwo.
The 3B20 was designed with the "rear midship" layout in mind. The basic dimensions chosen reduced the powerplant's height, the cylinder block's structure was simplified, a timing chain was adopted, modularized components were used for the oil and water pumps, engine mounts, and fuel system. The aluminum construction and lightweight parts and materials in the manifolds helped reduce the weight of the engine by 20 percent compared with its iron-block 3G83 predecessor, while gains were also seen in torque, fuel economy and emissions.