During Vietnam the US Army was looking for a convoy escort aircraft. They did not like the USAF solution of scrambling jet fighters when a convoy of trucks got into trouble. They wanted something overhead all the time.
Grumman proposed a COIN version of the Agcat crop duster. Based on the design of the F3F fighter, the airplane was simple, reliable, very rugged and resistant to ground fire, able to loiter for extended periods, and capable of accurately delivering considerable effective ordnance; the Army concluded it was just what they needed. "How many do you want to buy?" asked Grumman. The Army response was, "What? Us buy a biplane today? It may be exactly what we need but we'd be laughed out of the Pentagon!"
During Vietnam in 70-71 when I was there, the Army escorted convoys with OH-6s flying low, and Cobras at about 500 feet up as high escort in case anything big was found. They had been doing it for several years.
Plus the USAF was extremely reluctant to let the Army have any fixed wing aircraft beyond puddle jumpers. It was quite a struggle between the Army and the USAF over the Mohawk, and Bronco.
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