[1] When Messerschmitt was eventually allowed to start work on the project, designated the HA-300, he first built a tailless delta-winged glider, the Hispano HA P-300 (also known as the HA-23P).

[2] Due to funding problems, non-availability of the planned afterburning Bristol Orpheus engine for production and the resultant long development time, Spain abandoned the project in 1960.

[5] Ferdinand Brandner, an Austrian jet engine expert, was also invited to develop a turbojet for the new fighter, to replace the abandoned Orpheus.

Egypt aimed to produce a lightweight supersonic, single-seat fighter that could join the Egyptian Air Force as an interceptor.

The third and last prototype was fitted with the Egyptian E-300 engine, which it was hoped would make it capable of attaining 12,000 m (39,000 ft) and Mach 2.0 within 2.5 min after takeoff.

The HA-300 was modified for the Egyptian Brandner E-300 engine, whose planned output was to have been 10,600 lbf (47.2 kN) of thrust on afterburner, which would also improve the HA-300's performance.

The German engineers working on the design were forced to leave Egypt after receiving multiple death threats from the Israeli Mossad.