26 September 1936,
Francoist aviation bombed Bilbao. Six devices threw bombs on citizens twice. The day before also attacked the city with air and the two-day attacks resulted in 96 deaths and 650 injuries. Bilbao was first bombed in September 1936 and was attacked 40 times throughout the war.
Source:
Xabier Irujo, Atlas of Bombardments in the Basque Country (1936-1937) (Memory Collection).