A game that was supposed to be a defensive battle featuring two backup quarterbacks was anything but, as Colin Kaepernick led the 49ers to a 32-7 victory over the Chicago Bears on Monday in his first career start.

Kaepernick completed 16-of-23 passes for 243 yards and a pair of touchdowns, while Aldon Smith piled up 5½ sacks of Bears quarterback Jason Campbell, the second best single-game sack total in franchise history.

Both quarterbacks got the start after the starting signal callers — Alex Smith for the 49ers and Jay Cutler for the Bears — sustained concussions last week. Smith wasn’t officially ruled out until Monday morning. Coach Jim Harbaugh said Kaepernick did just about everything well filling in for him.

“I thought Colin did an outstanding job,” he said. “I thought he did a great job, poise in the pocket, pushing the ball, throwing with accuracy, did a nice job running the game plan. Everything he did was exemplary.”

The fact that the 25-year-old was making his first career start didn’t stop the 49ers from running the game plan they wanted, however, as they threw on two of their first three plays, including a 22-yard pass to tight end Vernon Davis that helped set up a field goal to give San Francisco the early lead.

“Our offense really isn’t going to change no matter who’s in there,” Kaepernick said. “Our offense is our offense and we’re going to run it.”

Kaepernick went to Davis early and often, as the tight end caught four of Kaepernick’s first nine completions, including a 3-yard touchdown that gave San Francisco a 10-0 lead with 6:19 left in the first quarter. Davis finished with 83 yards on six receptions. His touchdown was his first since he scored four times in the team’s first three games of the season.

Harbaugh said the early throws were part of the plan.

“We felt like two play-action passes to open the game, or at least two out of the three,” he said. “Thought maybe they wouldn’t expect that, thinking we were going to run with the backup quarterback.”

The Bears’ offense was not so versatile with Campbell at the helm, making his first start since getting hurt in Week 6 of last season while he was with the Raiders. Chicago ran on seven of its first nine plays and only crossed midfield twice — on a 13-play touchdown drive in the third quarter and its final drive of the game, in which it failed to convert a fourth-and-1.

The San Francisco defense kept the pressure on throughout the game, sacking Campbell a total of six times, forcing him to fumble three times, though he never lost any, and picking him off twice.

“Coming in, I think there was some questions about who may be the better defense,” Aldon Smith said. “Or just questioning us in general, but I think obviously we came out and proved a point tonight.”

Harbaugh said he decided Sunday night that Kaepernick would get the start, though he didn’t find out until Monday morning that Smith would be unavailable. The strong showing by Kaepernick left plenty of questions about who would start next week’s game in New Orleans.