Najmul ton guides Bangladesh to 235-2 against Afghanistan
Najmul led the charge after the hosts were sent in to bat on a green surface at the Sher-e-Bangla National Stadium in Dhaka.
He was 126 not out at the break after scoring his third Test century, while opener Mahmudul Hasan made his third fifty on his way to 76.
Former captain Mominul Haque was unbeaten on 11 at the interval alongside Najmul, who has hit 21 fours and one six so far.
Najmul and Mahmudul put on 212 runs in the second wicket partnership after they came together in the second over of the innings.
Occasional spinner Rahmat Shah broke their partnership when Mahumdul attempted a cut shot only to edge a catch to Ibrahim Zadran at slip.
Mahmudul hit nine boundaries in his sedate 137-ball innings.
Debutant pacer Nijat Masood was the only other Afghanistan bowler with any success, removing opener Zakir Hasan for one with his first Test delivery in the day's second over.
Zakir was initially declared not out by the on-field umpire but the replay indicated that the left-hander had nicked the ball to wicketkeeper Afsar Zazai.
Najmul hit back-to-back fours off Yamin Ahmadzai in the next over to deny Afghanistan further momentum.
A single off Amir Hamza earned him his hundred off 118 balls.
Afghanistan won the only Test between the two teams when they beat the home side by 224 runs at Chittagong in 2019.