The Eagle was the first engine to make a non-stop trans-Atlantic crossing by aeroplane when two Eagles powered the converted Vickers Vimy bomber on the Transatlantic flight of Alcock and Brown in June 1919 8:30.
At the outbreak of World War I in August 1914, the Royal Aircraft Factory asked Rolls-Royce to develop a new 200 hp air-cooled engine. Despite initial reluctance they agreed on condition that it be cooled by water rather than air, as this was the company's area of expertise.
Development of the new 20 litre engine was led by Henry Royce from his home in Kent. Based initially on the 7.4 litre 40/50 Rolls-Royce Silver Ghost engine, and drawing also on the design of a 7.2 litre Daimler DF80 aero engine used in a 1913 Grand Prix Mercedes that had been acquired, the power was increased by doubling the number of cylinders to twelve and increasing their stroke to 6.5 inches, although their bore remained at 4.5 inches of the 40/50. The engine was also run faster, and an epicyclic reduction gear was designed to keep the propeller speed below 1,100 rpm. To reduce inertia and improve performance the valvetrain design was changed from sidevalves to a SOHC design, closely following the original "side-slot" rocker arm design philosophy used on the contemporary German Mercedes D.I, Mercedes D.II and Mercedes D.III straight-six aviation powerplants.
On 3 January 1915 the Admiralty ordered twenty-five of the new engines. The Eagle first ran on a test bed at Rolls-Royce's Derby works in February 1915, producing 225 hp at 1,600 rpm. This was quickly increased to 1,800, then in August 1915 to 2,000 rpm where it produced 300 hp. After further testing, it was decided to approve the engine for production at 1,800 rpm and 255 hp; 1,900 rpm was allowed for short periods. The engine first flew on a Handley Page O/100 bomber in December 1915, the first flight of a Rolls-Royce aero engine.
The Eagle was developed further during 1916 and 1917, with power being progressively increased from 225 hp to then 266 hp, followed by 284 hp, and then 322 hp, and finally 360 hp by February 1918 by which time eight Eagle variants had been produced. Throughout World War I Rolls-Royce struggled to build Eagles in the quantities required by the War Office, but the company resisted pressure to licence other manufacturers to produce it, fearing that the engine's much admired quality would risk being compromised.