Instead of being paid to deliver planes, missiles and tanks, defense contractors would receive “weapon supply tax credits” (WSTC). The defense contractors would be able to reduce the taxes they owed the federal government by the prices of the weapons they delivered. Because the tax credit would be refundable, if the prices exceeded a firm’s annual tax liability, the IRS would send a check to the firm in the amount of the difference. In this way, the federal government could finance a massive military buildup — and because tax credits aren’t counted as part of the federal budget, for official purposes the cost of the buildup would be zero!
I had you going there for a minute, didn’t I? The “weapons supply tax credit” is a joke. It was proposed some years ago by the late David Bradford, a Princeton economist who worked in the Ford and George H.W. Bush administrations. Bradford’s purpose was to ridicule the growing reliance of Congresses and presidents on tax credits and other so-called tax expenditures as an alternative to ordinary spending programs funded by ordinary taxes.
From Taxes - salon.com

Excellent article on the misuse of Tax Credits as a substitution for taxes as a way to fund public goods.