SCOTTSDALE, Arizona—For so many SUVs, or would-be SUVs, diminishing their size has meant diminishing returns. At least, for customers. By the time you option up a bite-sized crossover, you realize it would have been smarter to move up one size, to an SUV that's demonstrably more useful for people and their proliferating stuff for similar money.

To combat the potential for such disappointment, the all-new 2020 Mercedes-Benz GLB250 takes a more right-angled approach to mall-SUV quadratics. Surprisingly practical for its size and semi-affordable, the GLB seems poised to lure new fans into Mercedes' luxury web. And Mercedes dealers stand ready with GLC, GLE, GLS and G-Class models for the day that GLB owners expand those families, incomes and expectations.

Count me among people who didn't imagine Mercedes could slice the SUV sausage any thinner. But it's the way they've sliced the GLB that makes you say, "Ah, I might actually go for one of those." Instead of a steep-sloping roof, the go-to for crossovers that want to suggest (often falsely) agility and spunk, the GLB flaunts the squarer, two-box proportions of old. It's a handsome sucker, too, in part because it presents itself with the honest, upright grace that once characterized most every truck-inspired SUV. Only this one's better than a Mitsubishi Montero.

Of course, Mercedes couldn't resist throwing some glitter at the clean-bodied GLC, from frosty "off-road" headlamps to such options as a bold diamond-pattern grille and 19-inch AMG wheels. Add Mercedes' dogged emphasis on ride comfort and low NVH, and a thick roster of luxury, tech and safety features — all impressive in an SUV that starts from $37,595 — and the GLB comes off as more a scaled-down GLS than a sportier, slightly smaller take on a compact GLC.

The imitation-by-flattery includes Mercedes' flashy new MBUX infotainment system, with its optional (and sure to be popular) conjoined, 10.25-inch display screens stretching from behind the steering wheel to the center of the dashboard. These ooze luxury, just as they do in far-pricier Benzes. (A lesser pair of 7.0-inch screens is standard, but you won't want those.) Welcome standard features include 18-inch wheels, LED headlamps, dual-zone climate control, keyless start, a power liftgate, Dynamic Select driving modes, eight-way powered front seats with position memory, a multi-function leather steering wheel, three-way split reclining rear seats, and Apple CarPlay and Android Auto. Add $2,000 for 4Matic all-wheel drive, which can now send up to 50 percent of engine torque to rear wheels, and the GLB becomes a legitimate four-seasons hauler. There's even an optional off-road package with hill descent control. Really.

The GLB seems to fall literally short of its big-brother GLS in one key area: The available seven-passenger version, versus the standard five-passenger model. As for the size of Row Three in this kleine German cabaret? Apparently, it's so small that even Mercedes couldn't locate one: Our press convoy of GLB's lacked any three-row specimen for journalists to sample, perhaps because they'd only make fun of it. (We're told that the company designed the pop-up third-row bench an occasional perch for children shorter than about five-and-a-half-feet tall.) But for an extra $850, families are welcome to pretend they bought a GLS that's 20 inches longer. At a certain point, size does matter.

We'd skip the three-row version. Note, however, that because the seven-passenger GLB exists at all, the room that setup requires benefits two-row GLBs all the more. This Mercedes feels the roomiest of all Tiffany-box luxury SUVs, including the Audi Q3, BMW X1 and Range Rover Evoque. The class-topping 111.4-inch wheelbase—the longest of Mercedes' entry-priced "gateway" models—and overall length both stretch about five inches beyond the German competition. Throw in that volume-boosting tall roof, and the GLB can actually swallow 10 percent more cargo than the pricier GLC, despite being an inch shorter overall. The 62 cubic feet of seats-folded-down cargo space easily tops the class, representing about 20 percent more than the Evoque's capacity, and 40 percent more than you'll find in Mercedes' smaller GLA crossover-ish hatchback.

Climbing aboard the GLB in Arizona, I was impressed by the open space between my noggin and the panoramic sunroof (a $1,500 option) above. There's 40.7-inches of front headroom, just a half-inch shy of the massive GLS, and it holds fairly steady as the roof tapers to the rear, including 39.3 inches in the second row. That reclining middle row pivots nearly nine inches fore-and-aft on seven-passenger models, or 3.7 inches on five-seaters. Legroom in the first two rows is on par with typical midsize SUVs and sedans, at a respective 41.4 and 38.1 inches. Next to the way-back seats' occupant height maximum (five-foot, six inches), they also squeeze potential riders with 29.1 inches of legroom and don't leave much space behind for cargo. Let's just say soccer dads won't be squeezing their balls in back.

The airy, inviting interior is a GLB high point. A dimpled, rounded lower dash supports a facing of (convincing) ersatz metal, topped with a formally flat, stitched dashboard. The MBUX screens nestle into a dashboard crevice, flashing their beautiful color graphics and endlessly reconfigurable displays. Natural-grain linden wood trim (a $325 option) further diminishes any traces of cost-cutting. And for $310, 64-color ambient lighting casts a moody glow in the cabin.

It was only a matter of time before someone imitated BMW's Gesture Control. Mercedes' take on it, the $300 Interior Assistant, performs tricks such as illuminating the console area when you reach for something in the dark. Less successful is the brand's Siri wannabe, a digital genie who you summon by saying "Hey, Mercedes," at your own Aladdin-esque peril. The digital helpmate garbled, misapprehended, or ignored roughly 6 of every 10 voice commands. And the assistant still annoyingly springs to life should you say the word "Mercedes" in normal conversation. But the coolest new bit is Augmented Reality for Navigation, which automatically beams a real-time camera view of upcoming turns or exits onto the windshield, and overlays it with directional arrows or street names. It worked great as we wound our way from Scottsdale through the Coconino National Forest and Western-themed towns.

Mercedes has joined Audi in ditching its trusty, familiar rotary control knob for the new MBUX unit, which is managed by a console touchpad, touchscreen, steering-wheel switches or voice prompts. (Yes, that's four ways to do what an analog button could handle, but apparently there's no going back). Lovely presentations aside, whether MBUX is easier and more intuitive than the old COMAND system remains open to debate. That's especially true for the MBUX touchpad, which isn't as wonky and unpleasant as Lexus' abominable unit, but still requires a surgeon's touch at times to operate while in motion, with the attendant eyes-off-the-road concerns.

Quibbles over the driving experience were harder to come by. On that all-day drive to the magical Red Rocks of Sedona, the GLB left us so at peace that we could almost believe in Sedona's Goop-approved, New-Agey wellness racket. With 221 horsepower and 258-pound feet of torque from its turbocharged, 2.0-liter inline four, the GLB slips from zero to 60 mph in a Mercedes-tested 6.9 seconds. Power seems just-right for this application, including smooth passes on Arizona's two-laners. Top speed is 130 mph. That engine sounds awfully good for such a small four-cylinder: Subdued-yet-substantial, with nary a hint of stress. A seven-speed, paddle-shifted automatic transmission performs its dual-clutch duties, and very well. Polished ride quality was boosted by a $990 set of adaptive dampers, and the hushed interior seemed on par with any GLC or C-Class. The GLB did short-shift incessantly, venturing no higher than 5,800 rpm, despite an official 6,500-rpm redline, even when I kept the transmission in manual paddle-shift mode and the accelerator pinned.

The GLB seems designed to tamp down aggressiveness, but it also showed a beautiful feel for the road, tracking down long canyon descents with ease at 90-mph and more. Steering has the signature on-center heft and sophistication of pricier Benzes. (This is of little surprise, given the GLB's architectural DNA is shared with the also-upscale new A-Class sedan.) The serenity was boosted with a $2,250 Driver Assistance Package, with every imaginable safety and semi-autonomous feature. That included Mercedes' first-rate Distronic adaptive cruise control, which adds its own wrinkle, a navigation-linked camera that adjusted our pace in anticipation of upcoming curves and other geographic features.

Don't tell the president, but the GLB is built in Aguascalientes, Mexico, alongside the A-Class sedan, and should reach showrooms in time for Christmas. Sweet labor deals aside, Mercedes further boosts profits with a familiar tactic: A range of extra-cost options. Our tester's included a $1,650 Premium Package whose features include the dramatic MBUX screens and Keyless Go proximity key; $1,450 worth of black leather; and an $1,150 Multimedia Package that included the Augmented Reality-enhanced navigation.

Thus larded, our GLB250 checked out at $49,050. As for lineup expansions, a full-fledged AMG 45 version seems unlikely, but a half-fledged AMG GLB35 seems inevitable for our market, with the same 302-hp four you'll find in the AMG CLA 35 sedan. Even in this opening iteration, this winning GLB250 seems laboratory-designed to fit the desires and lifestyles of aspiring families. Just as importantly, their lifestyles will fit in the GLB, even if it can be optioned up to one-size-up prices.

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