The youngster will not turn 17 until the end of June but has been a key player for Birmingham during the 2019-20 campaign, contributing four goals and three assists in 35 appearances in all competitions.
In the Championship, he has played all but one of his side's last 33 fixtures, which is incredible for a 16-year-old in any league at any level, let alone in one of, arguably, the most competitive divisions in the world.
United were strongly linked with the midfielder in January but were unable to complete a deal.
Bellingham was recently shown around United's training ground, which included a meet with the club's former manager Sir Alex Ferguson. The Red Devils look to be pulling out all of the stops, but Borussia Dortmund and Bayern Munich have also been strongly linked with a move.
United's desperation to sign the teenager has reportedly led them to consider offering him a £100,000-a-week contract at Old Trafford. For a player believed to be on a youth wage of £145 a week, that is some jump considering he is still developing as a person let alone a footballer.
Born in Stourbridge, West Midlands, Bellingham is the eldest son of Mark Bellingham, who was regarded as a profilic scorer in non-league football, allegedly netting in the region of 700 goals in approximately 850 appearances for 15 clubs. The youngster actually joined the Blues as an Under-8 and turned out for their Under-18 team at the age of 14, which is an indication of his early promise.
What's more, Bellingham played for Birmingham's Under-23 side at the age of 15 in October 2018. A year later, he became the club's youngest ever first-team player - breaking Trevor Francis's record in the process - by featuring against Portsmouth in the EFL Cup at the age of 16 years and 38 days.
He then became Birmingham's youngest-ever goalscorer at the age of 16 years and 63 days when he registered against Stoke City in the Championship on August 31, before finding the back of the net once again in his next outing against Charlton Athletic.
Capped at Under-15, Under-16 and Under-17 level by England, Bellingham has shown his versatility by operating in a number of different positions for Pep Clotet's side during the 2019-20 campaign. Indeed, he has played as a central midfielder, a left midfielder, a right midfielder, an attacking midfielder and even as a makeshift centre-forward against Reading back in December.
Back in October, Clotet told reporters: "He can go forward, he can defend, he is very complete and despite being only 16 he handles the physicality of the midfield very well.
"The only difference between a player who is 16 and a player that has a lot of years as a pro is the way they handle the load. He's handling it very well at the moment and he's a strong player so I don't see why he can't hack it."
Birmingham's academy manager Kristjaan Speakman, meanwhile, recently told BBC Sport: "He's from a tight-knit family who have protected and supported him through what has been a fast-paced journey to the first team.
"You can't adapt to first-team football as quickly as he has, at the level he has, without having some really good support mechanisms around you. I'm sure he would point to his family as his first port of call. The boy has got an incredible emotional intelligence and maturity for someone of his age."
For now, Bellingham does not have an agent; he is managed by his father, who will therefore have a key role in his son's next move, which is widely expected to occur in this summer's transfer window.
It is not too difficult to understand why United are determined to sign him, although Chelsea are also believed to be in talks over a potential summer deal.
The FA's former talent ID manager for England's Under-15s to Under-17s Daniel Dodds previously told The Athletic that Bellingham was 'as good as anything he has ever seen'.
United's transfer policy has certainly shifted over the last two transfer windows, and there is little doubting that Bellingham fits the brief when it comes to exciting young players looking to prove themselves.
There are a lot of hurdles for Ole Gunnar Solskjaer's side to overcome before they can call Bellingham their player, but it does appear that the club are determined not to let the talented youngster slip away.