DETROIT, Sept 25 (Reuters) - Ford Motor Co said on Thursday by year end it will add 1,200 jobs and a second shift to build the Transit commercial van at a plant in Missouri, making it the automaker's largest volume factory globally.

Strong demand for the vans, which began production in April as a replacement for the E-Series, drove the decision to add the shift, Ford said. The expansion is covered, however, by the company's $1.1 billion investment in 2011, a company spokeswoman said.

The additional jobs will push employment at the plant, which also builds the full-size F-150 pickup trucks, to more than 6,000 people. Ford added 2,800 jobs at the plant in the Kansas City suburb of Claycomo, Missouri, in 2013 and 2012 to support the 2015 Transit launch and increased production of the F-150.

The plant builds the F-150 on three shifts and will begin production of the more aluminum-intensive 2015 F-150 early next year. The new truck goes into production first at Ford's Dearborn, Michigan, plant later this year.

By year end, the plant's annual capacity will be about 500,000 vehicles, pushing it ahead of Ford's plant in Valencia, Spain, which has an annual capacity of 465,000 vehicles.

Ford has said it introduced the Transit in North America in part to save costs by sharing design and engineering work around the globe. Introduced in Europe in 1965, the Transit has been the top-selling commercial van in Britain for 49 years and it is sold in 118 countries on six continents.

The 2015 Transit went on sale in the United States in June and demand hit about 500 in July and almost 1,100 in August for a year-to-date total of just over 2,000.

The E-Series was first sold as the Econoline in the United States in 1961 and has been the best-selling van on the U.S. market for 35 years. E-Series vans and wagons still will be available and sold side by side with Transit until late 2014.

The additional jobs at the Kansas City plant mean Ford will have hired more than 14,000 hourly workers at its U.S. plants since 2011, far above the 12,000 required under the last labor deal with the United Auto Workers union. (Reporting by Ben Klayman in Detroit; Editing by Lisa Shumaker)