Found it!
Biondetti's proposed Grand Prix car from 1935
It was apparently to have a 60-degree UNSUPERCHARGED, AIR-COOLED V12 engine, with twin ohc, two valves per cylinder and three carbs. Bore and stroke are quoted at 79*100mm, engine weight just 265 kilos. The engine was to be based on 12 Norton motorcycle engines.
The car was never built. The British magazine Motor Sport confirmed that plans had been dropped in the January 1937 edition.