We find out the answer to your question at the end of Act I when Joe Keller tells "his" story to Ann Deever, Steve Deever's daughter and Joe's former partner on whom Joe has conveniently blamed the defective parts. Joe's life is built around the fiction that he did not know that the parts were defective and it was all Steve's fault, however at the end of the play he is forced to confront his self-made fiction and admit that the 21 pilots that died were "all his sons" and face his own guilt in the matter. However, at this stage in the play he reveals to us how he views what happened:
Every half hour the Major callin' for cylinder heads, they were whippin' us with the telephone. the trucks were hauling them away hot, damn near. I mean just try to see it human, see it human. All of a sudden a batch comes out with a crack. That happens, that's the business. A fine, hairline crack. All right, so - so he's a little man, your father, always scared of loud voices. What'll the Major say? - Half a day's production shot... What'll I say? You know what I mean? Human. So he takes out his tools and he - covers over the cracks. All right - that's bad, it's wrong, but that's what a little man does... That's a mistake, but it ain't murder.
Note how in this dialogue Joe does his best to make Steve look better than he actually does, emphasising the pressure they were under during war time and the way that Steve was a "little man" who wouldn't be able to face up to either the General or Joe for the faulty cylinders. This is how Joe presents the "truth" of what happened, and it is only gradually that the real truth is exposed.