A sleeve valve engine, its basic design was developed from the Bristol Perseus.

The Aquila was developed two years after the somewhat larger Perseus, both being sleeve valve designs.

The primary difference was in size, the Perseus being based on the 5.75 by 6.5 in (146 by 165 mm) cylinder used in the Mercury engine, while the Aquila used a new and smaller 5 by 5.375 in (127.0 by 136.5 mm) sized cylinder.

The result was a reduction in displacement from 1520 to 950 cubic inches (24.9 to 15.6 L).

This would have made it an excellent replacement for the Bristol Jupiter, which ended production at 590 hp (440 kW) three years earlier, but by this time almost all interest was on ever-larger engines.