another example of poor design was the F-4 Phantom, not the plane itself, which was used by USAF and USN and featured a great design, but the fact that it was designed with no internal gun since designers were convinced that air-to-air missiles had rendered guns obsolete.
That sounded great until Vietnam came along and it turned out the AIM-7 and AIM-9 missiles the F-4 carried had been designed to track big, slow moving bombers and were almost useless against fast turning north vietnamese fighters..
needless to say, every US fighter since then carries an internal gun.
of course, it was'nt just the US, the Russians and the French made the same mistake. It was just the Israelis who insisted on having a gun and had the french instal a 30mm cannon in the Mirages they bought. They were put to good use in the six-day war...
