At the end of the twenties several redesign projects were initiated at Junkers Flugzeugwerke for the replacement of early Junkers designs of the twenties. Several type numbers in the 50s range were allocated for these redesign studies.
The J54 design study was initiated by the Junkers sales departement in 1929, which had the demand for an improved Junkers A35/K53 military aircraft of 30% increased payload and range compared to the origin A35 design. Several design improvements for the A35 had already been performed with the Junkers J44 and Junkers J46 in 1927, which concentrated on engine selections.
Aside from new engine selections, i.e. Hispano Suiza, Renault, Bristol Jupiter VI, Mercury, Hornet or Siemens Sh20 with a final selection for the British Jaguar engine, the J54 changed the classic A25 cantilever monoplane design into a highlever monoplane.
At the same time the Junkers Ju52 was under developement and all Junkers developement capacity was put into the Ju52 project. Pohlmann's developement team was fully concentrated on the K47 and A50 designs as well and the J54 design was delayed for a possible first flight in September 1932. Due to the financial problems of Junkers Flugzeugwerke this project finally was cancelled without any mockup or prototype built ups in 1932.
All known Junkers documents refer the designator J54 to the A35/K53 replacement project. However, there is a K-designator K45. Most (not all) military K-designators were formed at Junkers by reversing the corresponding civil designator number (i.e. A35 - K53). Therefore the K45 designator of the military Ju52 might originally have been intended for an improved Ju52 under the designator J54. As the K45 was a seaplane version of the Ju52, it might be that the J54 was intended as a seaplane version of the Ju52, while the Ju52 originally was intended as a pure landplane. The same practise was used with the G38 and the J40 (two numbers between the land- and seaplane project). As the Ju52 was already under work, when the J54 replacement for the A35 was started, it might be, that the seaplane version of the Ju52 was initially planned as the J54/K45, but was reintegrated into the Ju52 project very early leaving the '54' open for further designators. Nothing of that is proved ... everything is pure speculation!