SFD or X-66A...

It's traditionally the US Air Force that assigns a suffix to experimental aircraft in the USA. Thus, the US Air Force  has designated the Sustainable Flight Demonstrator (SFD) as the new X-plane, joining a list of pioneering experimental aircraft that have radically shaped aeronautics across the Atlantic over the past 76 years, since the X-1 made its first flight in January 1946.

...Based on a converted MD-90 

In January 2023, NASA selected Boeing and its industrial team to lead the development and flight testing of a full-scale transonic shrouded-wing demonstrator aircraft (the TTBW) as part of the SFD program. Thus, the SFD program has been given the name X-66A, underscoring its status as a vitally important U.S. experimental aircraft program. This is the first X-plane specifically designed to help the U.S. achieve zero net greenhouse gas emissions from aviation. To build the X-66A, Boeing will convert an MD-90 into a full-scale demonstration aircraft featuring high aspect ratio wings supported by guy wires.

A program in the X line

Previous experimental aircraft from Boeing and some of its subsidiaries (Rockwell-North American, McDonnell Douglas respectively) have tested hypersonic speed (North American X-15, X-37), reusable spacecraft (X-41), unmanned combat aerial vehicles (X-45, X-46) and integrated fuselage flying wings (BWB), (X-48). Advanced technologies such as SFD are one of the key levers that Boeing believes will be needed to achieve the goal of zero net emissions for commercial aviation by 2050, in addition to the transition to renewable energy, operational efficiency and fleet renewal.