Unlike typical boxer engines, flat-twelve engines often employ a crankshaft configuration similar to a 180° V engine. In this design, pairs of pistons share a crankpin, resulting in a flat configuration but not a true boxer layout where each piston has its own crankpin. This engine type offers a lower center of gravity compared to a V12. However, their wider dimensions have limited their use in front-engined cars.
The first known flat-twelve engine was developed by Porsche in 1947 for the uncompleted Cisitalia Grand Prix racing car. This engine, designated the Porsche Type 360, was supercharged and had a displacement of 1.5 L. Although one unit was built and used for top-speed testing, the car never entered a race.
Ferrari introduced the Ferrari 512 F1 in 1964–65, which competed in several Formula One races powered by a 1.5 L flat-twelve engine. This car raced concurrently with the V8-engined Ferrari 158. Ferrari returned to the flat-twelve configuration in 1970 with the Ferrari 312B. Its successor, the Ferrari 312T, debuted in 1975 and was instrumental in Ferrari winning the Formula One constructors' championships from 1975 to 1979. The success of the 312T prompted other Formula One teams to develop their own flat-twelve engines, such as the 1979 Alfa Romeo 177. In 1990, Subaru developed the 1235 flat-twelve engine for an unsuccessful attempt to enter Formula One as an engine manufacturer.
Porsche introduced an air-cooled flat-twelve engine in its 1969 Porsche 917 sports prototype. This engine was an evolution of the earlier flat-eight, but it adopted a V12 crankshaft configuration rather than the boxer setup of its predecessor. The dominance of the Porsche 917 may have influenced Ferrari's decision to switch from V12 to flat-twelve engines for the Ferrari 312 PB, which competed from 1971 to 1973. Alfa Romeo also utilized flat-twelve engines in their sports prototype racing cars, the Alfa Romeo 33TT12 (1973–1976) and the Alfa Romeo 33SC12.
For the 1991 sports-prototype racing season, Mercedes-Benz replaced their twin-turbo V8 engine with a naturally aspirated flat-twelve in the Mercedes-Benz C291. This engine featured a unique cylinder-head design with exhaust ports positioned where intake ports are typically found, pointing upwards. The intake ports were located between the intake and exhaust camshafts, angled outwards from the vertical, a design choice made to allow for a lower chassis installation. The C291 did not achieve success, and Mercedes withdrew from sports-prototype racing after the 1991 season.
From 1973 to 1996, Ferrari incorporated flat-twelve engines into several production models. These included the Ferrari 365 GT/4 BB (1973–1976), the Ferrari 512 BB (1976–1981), the Ferrari 512 BBi (1981–1984), the Ferrari Testarossa (1984–1991), the Ferrari 512 TR (1991–1994), and the Ferrari F512 M (1994–1996).
During World War II, military vehicles were powered by flat-12 engines. The British Covenanter tank used a Meadows D.A.V flat-12 engine producing 340 hp (250 kW), while the Churchill tank was powered by a 350 hp (261 kW) Bedford flat-12 engine. Panhard EBR armored cars also utilized flat-twelve engines during the 1950s. In the United States during the 1940s, the Franklin Engine Company produced a flat-twelve aircraft engine known as the O-805-2.