PBY Catalina landing at NAS Jacksonville during WWII.
(official US Navy photo)
PB stands for “Patrol Bomber” and Y is the designation assigned to the manufacturer: Consolidated Aircraft. The PBY Catalina was the most widely used amphibious aircraft in World War Two. Manufactured in the US, many planes went via Lend-Lease to our allies.
While the US Navy called it the PBY, the British called it the Catalina and the Canadians called it the Canso. You often see this in aircraft names in World War Two. Our Allies would call planes received from Lend-Lease a different name than Americans used.