U.S. aircraft in Vietnam appear to have been extremely vulnerable to AAA

[Juha]

IIRC Republic had put some hardening or fire suppression system in F-105 but because USAF wanted to cut down the unit price they were dropped out during production. Not the first or last time when peace time economics became very costly during wartime.

Juha

[John Beaman]

Remember, also that the F-105 was designed as a tactical nuclear bomber, not a sustained iron bombing machine, hence infrastructure systems for a different concept.

The A-1 was designed in WWII as a tactical support machine and soldiered on long after it should have.

The F-4, in spite of upgrade attempts was designed as a Navy fleet intercepter for Soviet bombers, not a tactical bomber or even a dogfighter.

Mr. Johnson's war forced many planes into roles they were never conceived for or designed for. US pilots did an incredible job making them work, anyway.