Born in Iași, he graduated in 1915 first in his class at the Boarding High School of Iași. He then enrolled in the Faculty of Sciences at the University of Iași, but his studies were interrupted by the entry of Romania in World War I; conscripted into the army, he fought as a second lieutenant at the battles of Nămoloasa, Băltăreți, and Mărășești in the summer of 1917. General Henri Mathias Berthelot, the head of the French military mission to Romania, decided to send a group of young Romanians (including Hulubei) to France to train at an aviation school; upon completing the training, Hulubei participated as a pilot on a fighter aircraft of the French Air Force on the Western Front. Gravely wounded, he was awarded the Legion of Honour.