The civilian airframe behind the P-8 Poseidon

What civilian airframe is the P-8 Poseidon based on?

It is based on the Boeing 737 Next Generation family rather than a clean-sheet military airframe.

The P-8 is a military derivative of the Boeing 737 Next Generation. A tentative estimate puts parts commonality at roughly 40 percent, without treating the patrol aircraft as interchangeable with an airliner. Figures cited in the discussion put the 737 Next Generation fleet at 7,123 aircraft and the P-8 fleet at about 187. The much larger commercial fleet offers a broad parts and maintenance base for the specialised military aircraft. P-8 operation by allies including Norway, Germany, and the United States also provides a wider common support pool. These advantages strengthen the argument for retaining Britain's existing aircraft and infrastructure.