The 401 and 402 use an improved version of the BMW 328 hemispherical-head engine and its unusual arrangement of two separate pushrods to operate the exhaust valves, necessitated by the hemispherical combustion chambers and opposite facing valves, as used in the 400.

It was inspired by a Milanese designer, Carrozzeria Touring, and its most notable feature is that the door handles are not exposed.

They are also deeply curved at the front: this, along with the then-unique door handle arrangement, is believed to give the 401 a drag coefficient of less than Cd 0.36 – competitive even by today's standards and remarkable for the time.

[citation needed] The engine is the same 2-litre in-line six-cylinder petrol unit as in the 400, but upgraded with improved Solex carburettors to increase power by 5 bhp (3.7 kW) to 85 bhp (63 kW).

A fuel consumption of 20.8 miles per imperial gallon (13.6 L/100 km; 17.3 mpg‑US) was recorded.