The EF-131 was developed based on fragments of project documentation for the Ju 287 after the Red Army captured the Junkers factory in Dessau.
The V2 was nearly complete before the end of World War 2, but was hidden in the forest at Brandis along with Ju 287 V1 and eventually blown up by the Germans to avoid capture by US forces, and remnants of it, including wing sections, were taken into Red Air Force hands under military intelligence supervision along with the skeletal airframe of the unfinished V3.
[1] The V3 was to have been the first 287 to be made to pre-production model specifications, and the eventual EF-131 was almost identical to it in terms of overall design, except in having a slightly longer fuselage.
OKB-1 at GOZ-1 was formed with Dr. Brunolf Baade as the chief designer, and a very talented team of German engineers seconded by the Soviet government.
[2] The aircraft sat at LII over the winter but the harsh conditions caused the deterioration of rubber components and wiring, which required lengthy repairs.