Which straight-six aircraft engine was BMW's first product?

The BMW IIIa was BMW’s first product: a straight-six aircraft engine developed in 1917.

The engine was created by engineer Max Friz for BMW’s predecessor company, Bayerische Flugzeugwerke-era operations associated with Rapp Motorenwerke. Its six-cylinder layout was chosen for smooth running and good balance, while its approximately 19-liter displacement gave it the power needed for military aircraft of the First World War.

The IIIa became famous because it performed especially well at altitude. Its carburetor and high-compression design allowed it to maintain useful power where many contemporary engines lost performance. The engine powered aircraft such as the Fokker D.VII, one of Germany’s best-known fighters, and helped establish BMW as an aircraft-engine manufacturer before the company built cars.

The answer is easy to confuse with later BMW aircraft engines. The BMW 132 was a nine-cylinder radial engine, the BMW 801 was a much later fourteen-cylinder radial, and the BMW 003 was an axial-flow turbojet. None was BMW’s first product. The IIIa is also distinct from BMW’s later automotive engines because it was designed for aviation from the start.