The Dow Jones Industrials (DJINDICES:^DJI) climbed above the 16,000 mark for the first time this week, with the average taking just over six months to go from hitting the 15,000 milestone for the first time to its latest achievement. But when you look at the companies primarily responsible for the Dow's gains, you find four names that stand out: Boeing (NYSE:BA), Nike (NYSE:NKE), 3M (NYSE:MMM), and UnitedHealth Group (NYSE:UNH). Let's take a closer look at these four stocks to figure out why they had such a big impact on the Dow's advance.

Boeing has risen 45% since early May, when the Dow hit 15,000 for the first time. Bullish investors have pointed to the fact that Boeing expects the size of world aircraft fleet to double over the next 20 years, with the need for more than 35,000 new airplanes leading to spending of about $4.8 trillion on those planes. Although Boeing won't capture all of those sales, its share should be substantial. Moreover, with high fuel prices supporting demand for its more fuel-efficient models, Boeing can expect many airlines to upgrade existing fleets with new planes as soon as they become available.

Nike only joined the Dow in September, but the stock has risen 24% over the past six months, with a good part of those gains coming in the run-up to Nike's joining the average and its immediate aftermath. For Nike, fundamentals justified the move upward, with fiscal first-quarter results announced in late September showing the athletic apparel giant's rare ability to generate revenue growth in the North American market. Even with headwinds hurting its overseas results, Nike has persistently found ways to maintain high margins and enhance the value of its brand, reaping premium prices for its products even under less than ideal economic conditions.

3M is up 23% since May, and its big move this year is surprising given the company's conservative business strategy lately. Yet even though its rivals have been more aggressive about capturing opportunities in high-growth industries, 3M has still set the stage for impressive expansion of its business over the long run, and investors have recognized that fact. For many, 3M won't have proven itself until it comes up with another high-profile innovative product that everyone can point to as evidence of the company's inventiveness. In the meantime, though, steady growth and solid dividends should keep shareholders happy.

UnitedHealth has also gained 23% in the past six months, with the Affordable Care Act's impact front-and-center in the health insurance company's consciousness. Investors have been hopeful about the potential positive impact of Obamacare on UnitedHealth, but the company itself appears to be taking all available steps to avoid being overly reliant on the legislation's success. With steps like buying its stake in Brazil's Amil last year, UnitedHealth is trying to become a worldwide player in health insurance and reduce its political exposure to any one country.

Fool contributor Dan Caplinger has no position in any stocks mentioned. You can follow him on Twitter: @DanCaplinger. The Motley Fool recommends 3M, Nike, and UnitedHealth Group and owns shares of Nike. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.