Sources: The Motley Fool and Trex.

The American economy is growing stronger. Unemployment is at the lowest levels in almost a decade, the housing market is steadily bouncing back, and U.S. manufacturing is having a bit of a renaissance. The global economy is stalled a little, but much of Asia continues to grow, with the growth happening in the the consumer-driven middle class. In short, these look like sustainable, long-term trends.

We asked three Motley Fool contributors -- John Rosevear, Jason Hall, and Steve Heller -- for their thoughts based on these trends, and they came up with three industrials stocks that look set to be big winners in 2015: General Motors (NYSE:GM), Proto Labs (NYSE:PRLB), and Trex (NYSE:TREX). Will these three be the best industrial stocks for 2015? Let's take a closer look at why these Fools think so. 

Steve Heller: Zeroing in on an industrial stock that could outperform Mr. Market in 2015, I immediately gravitate toward Proto Labs as being a great candidate. The quick-turn manufacturer leverages cutting-edge technology and automation to deliver extremely fast turnaround times (often a matter of days) for product developers needing to produce tens of thousands of parts with real manufacturing processes like CNC machining and injection molding. On a high level, investors can think of Proto Labs as a Kinko's for manufacturing, except the barrier of entry for competitors to encroach on its turf is much higher, because the capital, manufacturing experience, and level of scale required doesn't come cheap.

Distilling it further, the strength of Proto Labs' business model is threefold. First, the breadth of manufacturing services that the company offers its customers is unprecedented for the price and turnaround time in the marketplace. Second, Proto Labs' relentless focus on customer service is geared toward creating long-lasting relationships from its growing base of product developers and serving their future needs. Third, its business model is highly adaptable, because the company is a technology implementer rather than a technology producer. This key difference means Proto Labs can simply acquire a disruptive manufacturing technology, like it did with 3D printing, and offer it as an added service to its customers.

Putting it all together, Proto Labs has been growing its revenues at a healthy clip over the last five years, indicating that the power of its model appears to be working:

PRLB Revenue (Quarterly) Chart

PRLB Revenue (Quarterly) data by YCharts.

Assuming Proto Labs can continue tapping into additional product developers -- of which there are hundreds of thousands worldwide -- for its quick-turnaround manufacturing services, I think the stock will continue rewarding investors over the long term.

Jason Hall: The housing bubble was a major contributor to the financial crisis that sent the global economy into a tailspin. However, the housing market has been steadily -- if with some fits and starts -- recovering over the past few years, as the economy has grown stronger:

^SPXTR Chart

^SPXTR data by YCharts.

However,  as you can see in the chart above, housing starts -- that is, new home construction -- still remain well below historical norms, and by many estimates, aren't at a level that will keep up with demand. There remains some oversupply in many markets, and it will take more time for that existing inventory to sell, but new home starts will continue to grow. 

However, Trex -- my pick for 2015 -- is more tied to existing home sales, and to the general state of the economy. And existing home sales have been showing some gains as well:

US Existing Home Sales Chart

U.S. Existing Home Sales data by YCharts.

But for Trex, it really does get back to the strength of the economy as the main driver of its success. Trex's business is manufacturing high-end deck products from recycled wood and plastics. The end product is beautiful and long-lasting, and doesn't require the annual maintenance a traditional deck requires, as the color is embedded in the material. No staining or waterproofing necessary means more time enjoying your outdoor space rather than maintaining it. 

Trex is easily the leader in its category, with more than one-third of the market share, strong brand recognition, and a solid leadership team beginning with CEO Ron Kaplan. Kaplan took over at Trex six years ago and has done a fantastic job of turning things around. The company is now larger and more profitable than it has ever been:

TREX Revenue (TTM) Chart

TREX Revenue (TTM) data by YCharts.

But with the housing market still on the rebound, there's huge upside. As the economy continues to grow stronger, more people will start spending on big-ticket items like Trex's decking products. It's a classic case of buying an industry leader on the right side of the demand cycle. 

John Rosevear: Want a great industrials stock that should have a good year in 2015? Take a look at General Motors.

No, I'm not kidding. You may look at the General and think "recall mess, lousy cars, lunch being eaten by Ford and overseas rivals" -- but the truth is a lot more interesting.

GM is now almost six years out of bankruptcy, it's under new management, and that new management is actually fixing General Motors. Their latest models are terrific: Models like the Chevy Impala, the Cadillac CTS, and the new Chevy Colorado pickup are winning awards left and right. Even better, they're winning praise in comparison tests in quarters where GM has, shall we say, not always fared so well.

What's that mean to the bottom line? In the auto business, it works like this: If your new cars and trucks are great, you can sell them with fewer discounts. That makes your profit margin fatter. If you factor out costs related to GM's huge recalls, its operating profit margin in the third quarter was 9.5% in North America -- better than Ford's (and a lot of other rivals').

GM CEO Mary Barra said recently that 2015 will be even better than 2014 -- even without taking those recall costs into account. And the story overseas is good, too: GM has lost a fortune in Europe over the last decade, but those losses will come to an end in 2016, Barra says.

And in China, where GM is rivaled only by Volkswagen for market share, GM's margins have been steady and strong -- and they'll get stronger over the next couple of years as the General adds more SUVs and luxury cars to its Chinese product line.

Here's the upshot: GM is doing better than most people think, and it's on track to do better still over the next couple of years. A (surprisingly) capable management team is making all of the right moves to fix GM's long-standing problems and inefficiencies, one by one. Many are already fixed -- the huge jump in the competitiveness of GM's products is proof of that.

Some on Wall Street have already caught on to the big changes going on at GM. Over the next year, I think a lot more investors will catch on -- and I expect GM's stock price to rise accordingly.