The winger, keen to move because he feared being on the fringes of the first team at West Ham , revealed that other top flight clubs were ready to sign him.
But he said: "They weren’t offering me anything different in terms of a challenge to West Ham.
“If I stayed at West Ham, the aim would be to try and push into the top half of the table.
“Last season went pretty well for the club and we finished 12th, next season the aim will be to improve on that.
“Had I gone to another Premier League club from West Ham, the targets would be similar, but at Boro there’s a chance to achieve something really special.
“This is probably the last move I’ll make in football so I want to try something I’ve never done before. It’s a great opportunity to help get the club back into the Premier League where they belong.
“Boro were very close to promotion last season and I thought if they need a few more experienced players to help them get over the line I could help.”
Downing, 30, rejoined Middlesbrough for £5m exactly six years to the day since he left them to join Aston Villa and he later played for Liverpool before joining West Ham two years ago.
“I feel like I’ve done something different, I’ve moved around the country and lived in different towns and cities. I’ve won a trophy and played in Wembley finals.
“At this stage in my career I’d rather play in a team that has a great chance of going up.”
Middlesbrough are also finalising a £3m deal for Uruguay international Christian Stuani, who scored ten times for Spanish club Espanyol last season.
The 28-year-old striker will link up with Middlesbrough next week at the end of a holiday taken after he played in the Copa America.