The British Army first took to the sky during the 19th century with the use of observation balloons. In 1911 the Air Battalion of the Royal Engineers was the first heavier-than-air British military aviation unit. The following year, the battalion was expanded into the Military Wing of the Royal Flying Corps which saw action throughout most of the First World War until 1 April 1918, when it was merged with the Royal Naval Air Service to form the Royal Air Force.