Boeing has had the EP-X, a development of the P-8 on the shelf since not long after P-8 development began.
It was initially proposed to replace the EP-3E Aries but this has been delayed by the USN. It was also designed so it could be configured for J-STARS, RJ and other missions as a cheaper alternative to the cancelled E-10 which was based on the 767-400 airframe.