Virgin America's firing back at James and Amber Marchese ... saying the 'Real Housewives' couple lied about a fight in which James allegedly choked Amber and even threatened to kill her.

Virgin has responded to the Marcheses' defamation lawsuit with a countersuit, claiming a flight attendant heard James tell Amber as they were boarding, "If you want to be on equal f****** playing fields then carry the f****** bags."

According to the countersuit, the flight attendant then saw James choke Amber, pulling her face close to him and threatening, "Bitch, if you do that again, I will f****** kill you."

According to the docs ... James then pushed Amber's face away and released his grip around her neck. A passenger also complained James appeared drunk.

James accused the flight attendant of being a fame whore who was making false allegations so she'd get attention, but according to the countersuit, the flight attendant didn't know who they were.

TMZ broke the story ... James was yanked off the flight and arrested for domestic violence. The charges were later dropped, and James claimed in his lawsuit he and Amber did nothing to warrant getting booted from the plane.