By 1933, Jacobs had developed its most famous engine, the L-4 seven-cylinder air-cooled radial, with a power rating of 225 hp (168 kW) displacement of 757.7 cu in (12,416 cc).

[6][7] Jacobs engines were fitted to many US-built aircraft of the inter-war period, including several Waco models.

They were in use in 26 different countries including in Canada, where 330 horsepower L6-MB engines were used to power the Royal Canadian Air Force's Avro Anson Mk.

[8] In 1941 the American War Department gave the contract to Jacobs to produce Pratt & Whitney R-985 and R-1340 engines until 1945.

[8] Jacobs ranked 87th among United States corporations in the value of World War II military production contracts.