See the second paragraph of my first answer to 

https://mathoverflow.net/questions/198466/ultraweak-topology-on-bx-is-the-map-x-otimes-x-bx-isometric

It references my paper

[J] Johnson, William B. A complementary universal conjugate Banach space and its relation to the approximation problem. Proceedings of the International Symposium on Partial Differential Equations and the Geometry of Normed Linear Spaces (Jerusalem, 1972). Israel J. Math. 13 (1972), 301–310 (1973),

although the particular case you want is due to Stegall (referenced in the above paper as well as in the Castillo-Gonzalez paper mentioned by Norbert).