Cancel: B-18, B-23 and B-26.
While it would be too late to cancel the B-18 prior to prototype stage, the performance of the Boeing 299 was so clearly superior to the Douglas DB-1 and Martin 146 competitors, the 1935 competition should have been cancelled when the Boeing entrant crashed due to human error (take off with control surfaces still locked). The B-18 was a dog from the start, with performance little better than the Martin B-10 it was to replace. The dissatisfied Air Corps wasted their money by trying to field an "improved" B-18, the B-23. If more resources could have been made available in the late '30s, there could have been more B-17 E and F models in service at the start of America's entry into the war. Another possibility might be the development of the B-38, an Allison V-1710 powered B-17, with roughly a 40 mph increase in top and cruising speeds and a whopping 1300 mile increase in range, to 3300 mi according to Wiki!
As for the B-26, it was envisioned to be a high speed medium bomber, but it's actual performance wasn't any better than the already adopted B-25 that operated with much less HP and was easier to fly. Rather than the B-26, it's too bad the USAAF didn't go for the B-28. At 370 MPH it would really have been a high speed medium bomber.