X-Plane 9.70+ McDonnell XF-88B 1.0. The first prototype XF-88 was modified to XF88B standard, with a nose-mounted Allison T38 turboprop engine added to the turbojets. This was used for flight testing through 1956, and achieved speeds slightly exceeding Mach 1.0 the first propeller-equipped aircraft to do so. McDonnell also proposed a naval version of the XF88, a two-seat operational trainer and a reconnaissance variant but none were built. Both prototypes were scrapped by 1958. By Vasco Ribeiro.