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The Martin RB-57 (a variant of the Martin B-57 bomber) and Lockheed U-2 were two impressive reconnaissance aircraft that entered service around the same time. In fact, the RB-57D model entered service shortly before the U-2 in order to fill a capability gap, and the later RB-57F variant remains in service today with NASA as a high altitude research platform.
The RB-57 has a much payload capacity than the U-2, as well as a higher speed and larger wing area. I think the U-2 has a higher altitude however, at 84,000 feet compared to around 82,000 feet for the RB-57. I'm wondering how it might have fared if it had been adopted more widely, filling the role originally intended for the U-2.