[...]
The P-3 came about as a response to Navy Type Specification #146 issued in 1957 for a new land-based antisubmarine warfare, or ASW, aircraft to replace the Lockheed P2V Neptune land-based maritime patrol aircraft and the Martin P5M Marlin flying boat. Very specific requirements pertaining to delivery schedule and cost constraints dictated the need for adapting an off-the-shelf aircraft design for the maritime patrol mission.
The competitors were Martin, Consolidated, and Lockheed, three companies that had been building patrol aircraft for the Navy for more than three decades at that point. The French Atlantique, developed with the help of US Navy funds, did not meet the stated range requirement and was eliminated from the competition.[...]