The earliest patrol aircraft carried bombs and machine guns. Between the wars the British experimented with equipping their patrol aircraft with the COW 37 mm gun. During the Second World War depth charges that could be set to detonate at a specific depth and later when in proximity with a metal object replaced "anti-submarine" bombs that detonated on contact.
Patrol aircraft also carried defensive armament which was necessary when patrolling areas close to enemy territory such as Allied operations in the Bay of Biscay targeting U-boats starting out from their base.
As a result of Allied successes with patrol aircraft against U-boats, the Germans introduced U-flak to escort U-boats out of base and encouraged commanders to remain on the surface and fire back at attacking craft rather than trying to escape by diving.