For decades, from the old Monsters of the Midway to the recent runs of Urlacher and Briggs, the Chicago Bears have been known for a dominant defense with a similar structure.

However, over the last two seasons Chicago has fielded one of the worst defenses in the league, which led the team to fall from playoff contender to NFL also-ran. New Bears head coach John Fox is aiming to change that in short order, and to do so he and new defensive coordinator Vic Fangio are taking the NFL’s archetype model for the base 4-3 defense and turning it upside down.

Chicago was one of the last remaining franchises to have stuck with the 4-3 with little to no alteration since the beginning. But those days are gone as Fox and Fangio have made it clear that the 3-4 will be their base defense of choice. It’s a surprising move, particularly for Fox who had success with the 4-3 in each of his other stops along the way to Chicago.

When he was hired it was assumed Fox would uphold Chicago’s 4-3 tradition. However when he brought in Fangio, a leading proponent of the 3-4, that assumption went out the window. Fox and general manager Ryan Pace confirmed as much in the wake of the Fangio hiring and then through the team’s personnel acquisitions in the last two months.

Since the beginning of the offseason, the Bears have added Jarvis Jenkins and Ray McDonald to their defensive front, and Pernell McPhee and Mason Foster to their linebacking corps. All four players are expected to be key contributors to the team’s scheme switch. But more important to the success or failure of Chicago’s defense will be how the team’s incumbent players adjust to the change, particularly All-Pro defensive end Jared Allen.

Over the course of his 11-year career, Allen has never played in a 3-4 defense, so things are about to get uncomfortable for him. The Bears have already noted that they will be moving Allen from his long-time position on the defensive line — where he has racked up 134 sacks, five Pro Bowl nods and four first-team All-Pro selections — to outside linebacker, a big adjustment for the 33-year-old veteran.

However both Pace and Fox believe it could be a beneficial switch for Allen, who had one of his worst seasons as a pro last year in his first season with the Bears, finishing with just 5.5 sacks in 15 games.

“I think he can be very flexible,” Pace said. “We can use him in a variety of ways. He’s done it for a lot of years. I feel the same way about several other players. [Lamarr] Houston is the same way. I think position flexibility is a strength of some of those guys. And again, I think Vic Fangio and John Fox are intelligent guys who will find ways to maximize their skill sets.”

“He could be one of those guys that might make a big jump,” Fox said of Allen after meeting with the veteran defender for the first time at the NFL owners meetings.

According to Fox, one of the advantages of the switch from the 4-3 to the 3-4 defense will be that it allows Chicago to disguise some of their pass rushing options. It will no longer be assumed that Allen is rushing from one end, allowing defenses to stack the line to take him away. Instead the offense will have to account for multiple rushers from each side, which the Bears believe will give Allen a better opportunity to get to the quarterback.

“The primary advantage of the 3-4 is you’re not locked into which outside guys are rushing,” Fox said. “That’s a tremendous advantage in today’s game. Even to the point where you’ve got guys that are a little bit comfortable in dropping, you can drop eight. With today’s spread offenses and some of the things people are doing with empty sets, that’s an advantage.”

Allen will also be part of a large scale rotation at the outside linebacker position. It’s likely that he’ll spend much of his time lining up opposite McPhee, who thrived in multiple positions in Baltimore’s 3-4 defense.

“He is a highly productive, disruptive and versatile defender. This guy not only brings the top physical talent but also the right mentality and makeup to improve our football team,” Pace said. “I like the violence that he plays with. He’s got length, gets off blocks. He’s a well-rounded player, too. He’s a productive pass rusher but also a steady, consistent run defender.”

The 26-year-old McPhee tallied 7.5 sacks for the Ravens last season despite not starting a game. Upon his arrival in Chicago, stated he’s ready to once again be a jack of all trades with the Bears under Fangio.

“I played defensive end in the 4-3, I played defensive end in the 3-4, and I played outside linebacker in the 3-4,” McPhee said. “We practiced against the 49ers last year, and their defense was very similar to us, so I think the scheme will be easy for me to adjust to. I look forward to helping some of the guys who might have a harder time adjusting to it.”

McPhee’s versatility gives Fox and Fangio some options. The team also has Houston set to return from a torn ACL and ready to switch back to outside linebacker after a failed transition to defensive end in his first season with Chicago. Houston’s 2014 campaign memorably came to an end in Week 8 of last season when he was injured celebrating his first sack of the year against the Patriots.

A big acquisition by the Bears last offseason, Houston knows he has a lot to make up for in year two, and he told the Chicago Sun-Times recently that the scheme change will put him in a better position to do that.

“Now I have the opportunity to go back to strictly playing outside linebacker and rushing on the edge,” Houston said. “I don’t have to necessarily put last year behind me because when I start playing this year, it’s going to be gone.”

Fox and Fangio will also have to figure out what to do with Willie Young when he returns from a torn Achilles suffered late in the season. Young was one the lone bright spots on the Bears defense last season. Another 2014 offseason acquisition by the Bears, he finished the season with 10 sacks from the defensive end spot. But Young doesn’t really have the size to play 3-4 DE. Instead, he’s likely to join Allen, Houston and McPhee in the outside linebacker rotation when he’s healthy, giving Fox and Fangio a multitude of pass rushers to work with.

Though they won’t have the middle linebacker star power that is synonymous with Chicago’s defense, the Bears are also deep at inside linebacker as the team makes the scheme switch. Now it’s just a matter of sorting out the depth chart between returning players and new acquisitions.

Jonathan Bostic, a second round pick in 2013, has been the Bears’ man in the middle for the majority of games the last two seasons, starting 17 of 32 games at middle linebacker, and he’ll have a good opportunity to lock down one of the inside linebacker spots. But he’ll have plenty of competition. With so many players already stacked up to compete at OLB, Fox has intimated that 2012 first round pick Shea McClellin is also likely to compete for a spot inside.

McClellin came out of Boise State as a 3-4 outside linebacker but when he arrived in Chicago, he went through a failed experiment attempting to mold him into a defensive end. He struggled again last season as a 4-3 outside linebacker, which has Fox ready to see what he can do as an inside backer with the versatility to move outside in the 3-4.

“It hasn’t gone as well for him as far as position fit,” Fox told ESPN at the NFL owners meetings.  “I think we’re going to start him inside because it’s a harder position to learn, but he will be both. So we don’t really have a position per se. A linebacker is a linebacker, and he’ll get a chance to compete at both. But more likely, we’ll start him inside.”

Also competing for a spot on the inside will be former Buccaneers linebacker Mason Foster, who joins the Bears after an injury-plagued final season in Tampa Bay. Prior to 2014, Foster looked like one of the league’s rising star middle linebackers, but the disappointing season forced Foster to take a one-year deal and he heads to Chicago with something to prove.

“My best football is definitely ahead of me. This 3-4 defense is going to give me a chance to make a lot of plays and use my skills to an advantage,” Foster told the Chicago Sun Times. “Everybody here has a fresh start – a new GM, new coaches. You want to get back to the way it was — the Monsters of the Midway.”

Undrafted free agent signing Christian Jones, who started five games at linebacker for the Bears last year and 2015 free agent signing Sam Acho also figure into the mix at inside linebacker giving Chicago another position of depth to work from.

The Bears don’t have as much depth up front and one of the priorities for Fox and Fangio over the next few months will be determining who will anchor the defense from the nose tackle position. The best in-house fit is defensive tackle Jeremiah Ratliff, who played the position in Dallas under Wade Phillips before Dallas changed to a 4-3 scheme. Last year, Ratliff impressed in 11 games with Bears, compiling 6.5 sacks from the 4-3 tackle spot.

But questions linger whether Ratliff has the size to be the anchor Chicago needs inside. That’s why many have tied the Bears, who own the No. 7 pick in the upcoming draft, to University of Washington nose tackle Danny Shelton. Chicago has monitored Shelton closely throughout the process leading up to the draft and he should be on the board when they pick.

“I can see myself playing over there,” Shelton told the Chicago Sun-Times. “Obviously the 3-4 teams, they have been going after me. I feel that I’m the No. 1 interior d-lineman that’s in this draft. I’m a hot commodity in a 3-4 defense.”

The addition of Shelton would be a tremendous fit, allowing the Bears to move Ratliff into more of a depth/rotational role while also putting them in better position to utilize the talents of bookend defensive ends, Ray McDonald and Jarvis Jenkins, two offseason signings aimed at beefing up the Chicago defensive line.

The Bears hope they can get more out of Jenkins than the Washington Redskins did over the first four years of his career. If not Jenkins is, at the very least, a big body to balance the line and provide a steady presence on the outside against the run.

McDonald is the bigger wild card for Chicago. The eight-year veteran had been a productive member of the San Francisco 49ers defense throughout his tenure, including for the last four seasons under Fangio. But after being accused of domestic violence in September and later of sexual assault in December, McDonald was released by San Francisco. McDonald notably has not been charged in either case, but the dual accusations have lingered over him.

Little interest came McDonald’s way in free agency and Bears ownership initially resisted bringing him in. But McDonald requested a meeting with Bears chairman George H. McCaskey, citing his relationship with Fangio. In that meeting, McDonald did enough to convince Chicago that he was not only wrongly accused, but also intent on staying out of trouble. The Bears gave him a one-year contract to prove it and if he can live up to his words, Chicago will have a big piece to their defensive line in place.

“It’s not a snap decision by any stretch. Like most players, a lot goes into the evaluation process. No different with him,” Fox said. “Obviously, any off-the-field issues we take serious. They do happen as much as you do like to keep them out of your building. You coach against it just like you do on the field. We felt strong about him, especially leaning heavily on four years, the relationship Ed Donatell and Vic Fangio had with him. And I think Ray understands we have a standard and he’s going to need to meet that standard moving forward, much like all the players we’ll acquire moving forward.”

While questions linger as to how the pieces will fit all over the defense, the Bears do have some solid pieces in the secondary. That group is led by last year’s first round pick Kyle Fuller. The rookie was another bright spot for Chicago last season, and should be even better in the man-to-man opportunities presented by the switch to the 3-4. Fox noted that having a player of Fuller’s talent in the secondary will only make everyone else on the defensive side of the ball better.

“The way he played instinctively as well as the physical skills. I saw a guy who improved as the season went on. It’s a big leap, especially at that position,” he said. “The passing game is a little more precise at our level. He’s a guy we’re excited about, have a good grade on.”

At the other cornerback spot, Chicago has the steady presence of nine-year veteran Tim Jennings. The Bears also added Alan Ball in free agency to strengthen their depth at the position. Behind those two and Fuller, helping to hold down the secondary will be one of Chicago’s biggest offseason acquisitions, strong safety Antrel Rolle. He’ll be paired with free safety Brock Vereen and the two provide some security at the backend of the Bears defense. The blend of veterans and young players in the secondary suits what Chicago is trying to accomplish according to Fox.

“I think leadership is important,” he said, “There is kind of a 10-80-10 percentage on a football team. There is 10 percent of the guys that are doing it right all the time. The key, though, is to get that 10 to go down and get the majority of that 80 to see it being done right so they can climb up to the 10. We think [Rolle] is in that 10 and has the ability and has done it before. I’ve seen him at the Pro Bowl even show leadership, which sometimes even in that game you need it.”

By way of new personnel and the new scheme, change is coming to the Chicago defense, the likes of which has been seen at Soldier Field and Halas Hall for a long time. But in many ways, it’s change for the better. The Bears now have a plethora of talented defensive players in place and the scheme change will give many of them a fresh start and golden opportunity to seize one of the many open spots. The competition alone is likely to breed strong results. Fox can see it all fitting into place, and as a man with a knack for fixing what’s broken, he’s up for the challenge of overhauling and revitalizing the hallowed defense of the Bears.

“I was in old Halas Hall a long time ago as a young college coach trying to learn more about defense…they have a tremendous tradition. You just walk around our building and it’s a who’s who of historical football,” he said recently. “I kind of love defense too. It’s a vision for us.”