After a dismal 2015 campaign marked by horrendous performance and reliability, Renault enjoyed an indisputable upswing last year with Red Bull, a partner it had almost severed ties with at the height of the French manufacturer's under-performance.
Bell is convinced Renault is on the right path, and while still a length behind Mercedes, it has caught up with Ferrari.
"It's certainly behind Mercedes, there or thereabouts with Ferrari, maybe slightly off but not by much," the British engineer told Motorsport.com.
"We have to close that gap and we'll make another step over winter. But whether that will see complete closure or not, you just can't tell. There's no way of knowing.
"We believe we can, given time. It might possibly not be this winter but certainly by the end of next year and into 2018, we ought to be right on it."
While Renault putting its performance back in line with the norm last year was taken for granted, Bell insists the effort was huge and the company should be praised for it.
"It is massively impressive what they've achieved, because they really set out with the mandate to make sure at the very least the engine was absolutely rock solid in terms of reliability, and they've achieved that," he said.
"It was probably the most reliable engine on the grid last year and that's a huge credit to them, but that hasn't stopped them putting a lot of performance in the engine."