Designed by A. A. "Alf" Fletcher in mid-1915, the Martinsyde G.100 was conceived as one of the first single-seat long-range escort fighters in the United Kingdom. The prototype first flew in August 1915, though some records indicate it entered testing in the autumn of that year using a 120 hp Austro-Daimler engine. The production G.100 was powered by a 120 hp six-cylinder, water-cooled inline engine produced by William Beardmore & Co. Ltd. Due to its significant size and weight, the Royal Flying Corps (RFC) eventually reclassified the aircraft as a light day bomber, as it lacked the agility required for effective dogfighting.
Production took place in the United Kingdom at Martinsyde Ltd's facilities in Brooklands and Woking, Surrey. The company, founded in 1908 as Martin & Handasyde, produced the G.100 and its uprated G.102 variant between 1916 and 1917. While production numbers vary, records indicate more than 100 G.100s and 171 G.102s were built, with total combined production estimated between 270 and 300 aircraft. The G.100 featured a wingspan of 11.58 meters and a length of 8.23 meters. It was capable of reaching a maximum speed of 153 km/h and a service ceiling of approximately 4,270 meters. Its most notable technical attribute was its endurance, allowing for flights of up to 5.5 hours.
Operationally, the G.100 served extensively during the First World War. No. 27 Squadron RFC was the only unit completely equipped with the type, deploying to the Western Front on March 1, 1916. Other units, including Nos. 18, 20, 21, and 23 Squadrons, utilized the aircraft as escorts. Beyond France, the "Elephant" saw service in the Middle East, specifically in Palestine and Mesopotamia, where Nos. 30, 63, and 72 Squadrons operated. In the Baku operations of 1918–1919, a detached flight of No. 63 Squadron RAF used the aircraft for reconnaissance over Turkish positions.
Armament typically consisted of one 0.303 in Lewis gun mounted on the upper wing, with later provisions for a second rear-firing gun. The aircraft could carry a bomb load of up to 260 lb, often configured as two 112-lb bombs. Despite its failures as a fighter, the G.100 was valued for its stability in photo-reconnaissance and long-range bombing. Martinsyde Ltd eventually ceased aviation activities and went into liquidation in 1922 following a factory fire. No original G.100 or G.102 airframes are known to survive today.