Buy American Act
This act provides a 6-12% subsidy to American suppliers; or, more accurately, a 6-12% inefficiency, compared to foreign suppliers, in addition to any applicable duties/tarriffs/etc.
Here's some information about the contracts awarded to major corporations:
linky. Now this does not specifically state that Boeing was awarded sole source contracts during the data period. Based on the overall value of contracts, and the number awarded as sole source, however, concluding that Boeing likely did not receive any such contracts seems dubious. In any case, McDonnell Douglas seems to have been awarded many more contract dollars than Boeing.
I've worked in government, and in contracts (in fact, at the same time) - I understand why many contracts, especially for consultants, are not put out for bid: the evalutation process costs more than ANY potential savings from a lower bidder. Billion dollar construction contracts are not candidates for such an explanation, however.