The 100 largest business-to-business advertisers spent an estimated $4.8 billion on U.S. b-to-b ads last year, up 0.9% from 2013, according to Ad Age Datacenter's analysis of measured-media spending from Kantar Media.

Estimated 2014 U.S. measured-media spending for the universe of b-to-b advertisers -- including the BtoB 100 and all other b-to-b advertisers -- was $10.2 billion, flat over 2013 spending. The top 100 b-to-b advertisers accounted for nearly half of all b-to-b spending last year.

Figures include estimated b-to-b ad spending across TV, internet (display ads only), magazines (including business publications), newspapers, radio and outdoor.

The largest media category for b-to-b ad spending last year was TV, with the BtoB 100 spending an estimated $2.9 billion on U.S. b-to-b TV ads. That was up 7.7% over 2013 and made up 60.8% of all b-to-b advertising.

The second-largest category was internet advertising, with the BtoB 100 spending about $685 million on b-to-b display ads last year, down 2.7% from 2013, and making up 14.3% of all b-to-b ad spending.

The third-largest category was magazines, where the BtoB 100 spent approximately $585 million, down 14.4% from 2013. Magazines composed 12.2% of b-to-b ad spending.

The top 100 b-to-b advertisers on average decreased spending on newspaper ads (down 15.8% from 2013) and outdoor (down 9.7%), although they increased spending on b-to-b radio ads by 7.5%.

The No. 1 U.S. b-to-b advertiser last year was Microsoft Corp., which spent an estimated $232.4 million on b-to-b ads. Despite being the largest b-to-b advertiser, Microsoft decreased its b-to-b ad spending by 19.9% from 2013.

Microsoft launched an integrated campaign last year pitting its mobile devices -- including the Lumia 635 smartphone from Nokia (which Microsoft owns) and its Surface Pro 3 tablet -- squarely against Apple's iPhone and MacBook Air.

"We are a challenger brand in the mobile category," said Kathleen Hall, general manager-global and brand marketing at Microsoft, in an interview with Ad Age last year about the integrated ad campaign.

The campaign was created by M:United, a new agency formed after Microsoft shifted its media and creative businesses to Interpublic Group and DentsuAegis last year.

The No. 2 b-to-b advertiser last year was AT&T, which spent an estimated $204.6 million on b-to-b ads, up 3.9% over 2013.

Last year, AT&T rolled out a brand platform, "Mobilizing Your World," to show how its mobile network helps businesses and consumers succeed. The campaign launched with a 60-second TV spot, created by BBDO New York, showing how AT&T technologies allow people to work seamlessly together.

The No. 3 b-to-b advertiser last year was Verizon Communications, which spent an estimated $168.6 million on b-to-b ads, down 8.8% from 2013. Verizon launched a TV campaign, created by McGarryBowen, showing how business people and consumers can be heroes by using Verizon's XLTE mobility network.

The remaining top-10 b-to-b advertisers, in order of spending, were Apple, Samsung Electronics Co., IBM Corp., General Electric Co., Intel Corp., Staples and Deutsche Telekom (T-Mobile).