When war broke out in 1914, Boutet de Monvel was called up as a reservist. He was injured during the Battle of the Marne. After a brief recovery he joined the 4th Bombardment Group as a bombardier. When the group disbanded in November 1915, he was appointed to the Orient Bombardment Group, based in Salonika, Macedonia. In June 1916, when this group disbanded in turn, he joined a new squadron, the C389. In September 1916, he and his pilot flew from Salonika to Bucharest, a feat that earned recognition in both France and Romania.