Based on the Boeing 737-800 airliner, the P-8 Poseidon (formerly the Multi-mission Maritime Aircraft or MMA) has been developed by Boeing Defence, Space & Security as a replacement for the United States Navy's (USN) aging fleet of Lockheed P-3 Orion aircraft which have now performed the role as an anti-submarine warfare (ASW), anti-surface warfare (ASuW) and maritime patrol / surveillance / interdiction platform for a number of decades and due for retirement.
Of interest, the Royal Australian Air Force (RAAF) will begin replacing it's fleet of Lockheed P-3C Orion aircraft with a number of P-8A Poseidon's and high-altitude unmanned aerial vehicles in the near future.