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05/31/2008. A prototype XPBY-4A and 32 PBY-4As were ordered on December 18, 1937 and the first flew in May 1938. The PBY-4A was the first Catalina model to have the large transparent machine gun blisters, supplanting the sliding hatches. The photo was taken after July 1, 1939, when a new Patrol Squadron numbering system was introduced, the '13' indicating this aircraft was assigned to the third squadron of Patrol Wing One.