O-2TT prototype [demonstrator]. A turboprop version of the O-2A was proposed by Cessna to the USAF for Viet Nam FAC/Liaison mission, late 1968, just after introduction of the piston engine O-2A [C337 variant] into its combat role. This version would have a significant performance and payload boost; would have had a wider mission capability; and would use USAF standard JP-4 fuel (not scarce av-gas).The TT designation indicated that both engines were turbine... Allison C250-B15 [in tractor and pusher configurations]. As usual, the USAF was not prone to adopting small/unglamorous prop birds that could fill the low-end combat roles very effectively and cost-efficiently... when every 'true fighter pilot' wanted to fly fast/big jet fighters that were capable of 'going Down-Town [Hanoi]'! Hence the O-2TT proposal fizzled... too bad, considering there was 6-more years of fighting up/down South Viet Nam.