Union says documents bolster NLRB case vs Boeing

* IAM says Boeing internal documents reinforce NLRB case

* Documents part of evidence to be entered in case

* Boeing declines comment

SEATTLE, Sept 23 (Reuters) - The union representing Boeing Co (BA.N) workers in Washington state on Friday said internal company documents show Boeing intended to punish union members for past strikes when it located an airplane production line in non-union South Carolina.

Connie Kelliher, a spokeswoman for the International Association of Machinists, said the documents show Boeing had determined years ago that a South Carolina 787 assembly plant was more likely to fail than other potential locations.

Nevertheless, the plane-maker chose South Carolina, a state unfriendly to labor unions, to avenge past legal strikes and avoid future strikes, the union said.

Boeing and the National Labor Relations Board are locked in a dispute over the location of Boeing's second 787 Dreamliner factory in South Carolina.

The case is being heard in Seattle by an administrative law judge but has drawn the attention of politicians who say the NLRB, dominated by Democrats, is impeding Boeing's right to run its operations as it sees fit.

The documents were part of the evidence subpoenaed by the NLRB in its complaint against Boeing, Kelliher said. The IAM distributed the documents by e-mail and to reporters at a union event in Seattle.

Boeing declined to comment on the documents, but has said before that it was never meant to punish unionized workers. The plane-maker further argues that it has not moved existing work from Washington, but instead created new jobs in South Carolina.

The 787 Dreamliner, which is set for first delivery next week, is about three years behind schedule. The company blames a 58-day labor strike by IAM members for part of the delay.

The company plans to building seven 787s per month at its Everett, Washington, plant and another three in South Carolina.

(Reporting by Karen Johnson; Writing by Kyle Peterson; Editing by Richard Chang)