Francillon’s epic McDonnell Douglas Aircraft Since 1920, Vol. 1 (Putnam 1988) states the plane only got as far as a design concept before being sold to Piaggio.
Since Douglas never built a prototype, I don’t think it competed against the Sabreliner (Sabre is the fighter) for the USAF UTX contract…issued in 1956.
In fact, it didn’t fly until 1964, long after the T-39 (ff 1958) had been bought by the USAF and brought to the civilian market.
Like the DH 125, it was powered by Vipers.
Twenty-nine were built, 25 Italian military aircraft, two prototypes and two civil examples.