During the First World War, the month of April 1917 was known as Bloody April by the Royal Flying Corps (RFC). The RFC suffered particularly severe losses - about three times as many as the Imperial German Army Air Service (Luftstreitkräfte) over the same period - but continued its primary role in support of the ground offensive.
In April, the Allies launched a joint ground offensive, with the British attacking near Arras in Artois, northern France, while the French Nivelle Offensive was launched on the Aisne. Their air forces were called on to provide support, predominantly in reconnaissance and artillery spotting.
The Battle of Arras began on 9 April 1917. In support, the RFC deployed 25 squadrons, totalling 365 aircraft, about a third of which were fighters (or "scouts" as they were called at the time). There were initially only five German Jastas (fighter squadrons) in the region, but this rose to eight as the battle progressed (some 80 or so operational fighter aircraft in total).
Since September 1916, the Germans had held the upper hand in the perpetual contest for air supremacy on the Western Front, with the Albatros D.II and D.III outclassing the British and French fighters charged with protecting the vulnerable B.E.2c, F.E.2b and Sopwith 1½ Strutter 2-seater reconnaissance and bomber machines. The allied fighter squadrons were equipped with obsolete 'pushers' such as the Airco DH.2 and F.E.8, and other outclassed types such as the Nieuport 17. Only the SPAD S.VII, Sopwith Pup and Triplane could compete on equal terms with the Albatros, but these were few in number and spread along the front. The new generation of Allied fighters were not yet ready for service, although No. 56 Squadron RFC with the S.E.5 was working up to operational status in France. The Bristol F2A also made its debut with No. 48 Squadron during April, but lost heavily on its very first patrol, with four out of six shot down in an encounter with five Albatros D.IIIs of Jasta 11, led by Manfred von Richthofen.
During April 1917, the British lost 245 aircraft, 211 aircrew killed or missing and 108 as prisoners of war. The German Air Services lost 66 aircraft from all causes. As a comparison, in the five months of 1916's Battle of the Somme the RFC had suffered 576 casualties. Under Richthofen's leadership, Jasta 11 scored 89 victories during April, over a third of the British losses.
The month marked the nadir of the RFC's fortunes. However, despite the losses inflicted, the German Air Service failed to stop the RFC carrying out its prime objectives. The RFC continued to support the army throughout the Arras offensive with up-to-date aerial photographs, reconnaissance information and harassing bombing raids. In spite of their ascendency, the German squadrons continued to be used defensively, flying for the most part behind their own lines. Thus the Jastas established "air superiority", but certainly not air supremacy.
Within a couple of months the new technologically advanced generation of fighter (the SE.5, Sopwith Camel, and SPAD S.XIII) entered service in numbers and quickly gained ascendency over the over-worked Jastas. As the fighter squadrons became able to once more adequately protect the slower reconnaissance and artillery observation machines, RFC losses fell and German losses rose.
This was essentially the last time that the Germans possessed real air superiority for the rest of the war - although the degree of allied dominance in the air certainly varied.